 Parity of zero

Zero is an even number. In other words, its parity—the quality of an integer being even or odd—is even. Zero fits the definition of "even number": it is an integer multiple of 2, namely 0 × 2. As a result, zero shares all the properties that characterize even numbers: 0 is evenly divisible by 2, 0 is surrounded on both sides by odd numbers, 0 is the sum of an integer with itself, and a set of 0 objects can be split into two equal sets.
Since definitions can change, another approach is to set them aside and consider how zero fits into the patterns formed by other even numbers. The parity rules of arithmetic, such as even − even = even, require 0 to be even. Zero is the identity element of the group of even integers, and it is the starting case from which other even natural numbers are recursively generated. Applications of this recursion from graph theory to computational geometry rely on zero being even. Not only is 0 divisible by 2, it is divisible by every integer. In the binary numeral system used by computers, it is especially relevant that 0 is divisible by every power of 2; in this sense, 0 is the "most even" number of all.
Among the general public, the parity of zero can be a source of confusion. In reaction time experiments, most people are slower to label 0 as even than 2, 4, 6, or 8. In schools, both students and teachers often hold misconceptions that zero is odd, or both even and odd, or neither. Researchers in mathematics education propose that these misconceptions can become learning opportunities. Studying equations like 0 × 2 = 0 can address students' doubts about calling 0 a number and using it in arithmetic. Discussing the issue in class can spark debates between students, during which they encounter basic principles of mathematical reasoning, such as the importance of definitions. Understanding zero is one goal, but there is also a wider lesson. Evaluating the parity of this exceptional number is an early example of a pervasive theme in mathematics: the abstraction of a familiar concept to an unfamiliar setting.
Contents
Why zero is even
It is easy to directly prove that zero is even:
 A number is called even if it is an integer multiple of 2. Zero is an integer multiple of 2, namely 0 × 2, so zero is even.^{[1]}
This proof starts with a standard definition of "even number". It is also possible to explain why zero is even without referring to formal definitions.^{[2]} The following explanations make sense of the idea that zero is even in terms of fundamental number concepts. From this foundation, one can provide a rationale for the definition itself—and its applicability to zero.
Basic explanations
A basic usage for numbers is counting. Given a set of objects, one uses a number to describe how many objects are in the set. Zero is the count of no objects; in more formal terms, it is the number of objects in the empty set. The concept of parity is used in making groups of two objects. If the objects in a set can be marked off into groups of two, with none left over, then the number of objects is even. If an object is left over, then the number of objects is odd.^{[4]}
The empty set contains zero groups of two, and no object is left over from this grouping, so zero is even. Although it is difficult to depict zero groups of two, or to draw attention to the nonexistence of a leftover object, this conception of the evenness of zero can be illustrated by comparing the empty set with other sets, as in the diagram on the right.^{[5]}
The number line provides a more uniform way of depicting numbers, including positive numbers, negative numbers, and zero. When even and odd numbers are distinguished visually, their pattern becomes obvious:
The even and odd numbers alternate. Starting at any even number, counting up or down by twos reaches the other even numbers, and there is no reason to skip over zero.^{[6]}
Parity can be approached in a more formal way using arithmetic expressions. Every integer is either of the form (2 × ▢) + 0 or (2 × ▢) + 1; the former numbers are even and the latter are odd. For example, 1 is odd because 1 = (2 × 0) + 1, and 0 is even because 0 = (2 × 0) + 0. Making a table of these facts then reinforces the number line picture above.^{[7]}
Defining parity
The precise definition of any mathematical term, such as "even" meaning "integer multiple of two", is ultimately a convention. Unlike "even", some mathematical terms are purposefully constructed to exclude trivial or degenerate cases. Prime numbers are a famous example. The definition of "prime number" has historically shifted from "positive integer with at most 2 factors" to "positive integer with exactly 2 factors", with the effect that 1 is no longer considered prime. Most authors rationalize this shift by observing that the modern definition more naturally suits mathematical theorems that concern the primes. For example, the fundamental theorem of arithmetic is easier to state when 1 is not considered prime.^{[8]}
It would be possible to similarly redefine the term "even" in a way that no longer includes zero. However, in this case, the new definition would make it more difficult to state theorems concerning the even numbers. Already the effect can be seen in the algebraic rules governing even and odd numbers.^{[9]} The most relevant rules concern addition, subtraction, and multiplication:
 even ± even = even
 odd ± odd = even
 even × integer = even
Inserting appropriate values into the left sides of these rules, one can produce 0 on the right sides:
 2 − 2 = 0
 −3 + 3 = 0
 4 × 0 = 0
The above rules would therefore be incorrect if zero were not even.^{[9]} At best they would have to be modified. For example, one test study guide asserts that even numbers are characterized as integer multiples of two, but zero is "neither even nor odd".^{[10]} Accordingly, the guide's rules for even and odd numbers contain exceptions:
 even ± even = even (or zero)
 odd ± odd = even (or zero)
 even × nonzero integer = even^{[10]}
Making an exception for zero in the definition of evenness forces one to make such exceptions in the rules for even numbers. From another perspective, taking the rules obeyed by positive even numbers, and requiring that they continue to hold for integers, forces the usual definition and the evenness of zero.^{[9]}
Mathematical contexts
Countless results in number theory invoke the fundamental theorem of arithmetic and the algebraic properties of even numbers, so the above choices have farreaching consequences. For example, the fact that numbers have unique factorizations means that one can determine whether a number has an even or odd number of distinct prime factors. Since 1 is not prime, nor does it have prime factors, it is a product of 0 distinct primes; since 0 is an even number, 1 has an even number of distinct prime factors. This implies that the Möbius function takes the value μ(1) = 1, which is necessary for it to be a multiplicative function and for the Möbius inversion formula to work, and affects the exact value of the Mertens function everywhere.^{[11]}
Not being odd
The observation that zero is not odd is sometimes directly applied in a mathematical argument. If an unknown number is proven to be odd, then it cannot be zero. This apparently trivial observation can provide a convenient and revealing proof explaining why a number is nonzero.
A classic result of graph theory states that a graph of odd order always has at least one even vertex. (Already this statement requires zero to be even: the empty graph has an even order, and an isolated vertex is even.)^{[12]} In order to prove the statement, it is actually easier to prove a stronger result: any oddorder graph has an odd number of even vertices. The appearance of this odd number is explained by a still more general result, known as the handshaking lemma: any graph has an even number of vertices of odd degree.^{[13]} Finally, the even number of odd vertices is naturally explained by the degree sum formula.
Sperner's lemma is a more advanced application of the same strategy. Rather than prove that there exists a completely labeled subsimplex by a direct construction, it is more convenient to prove that there exist an odd number of such subsimplices through an induction argument.^{[14]} A still stronger statement of the lemma then explains why this number is odd: it naturally breaks down as (n + 1) + n when one segregates colorings by orientation.^{[15]}
Evenodd alternation
The fact that zero is even, together with the fact that even and odd numbers alternate, is enough to determine the parity of every other natural number. This idea can be formalized into a recursive definition of the set of even natural numbers:
 0 is even.
 (n + 1) is even if and only if n is not even.
This definition has the conceptual advantage of relying only on the minimal foundations of the natural numbers: the existence of 0 and of successors. As such, it is useful for computer logic systems such as the Isabelle theorem prover.^{[16]} With this definition, the evenness of zero is not a theorem but an axiom. Indeed, "zero is an even number" may be interpreted as one of the Peano axioms, of which the even natural numbers are a model.^{[17]} A similar construction extends the definition of parity to transfinite ordinal numbers: every limit ordinal is even, including zero, and successors of even ordinals are odd.^{[18]}
The classic point in polygon test from computational geometry applies the above ideas. To determine if a point lies within a polygon, one casts a ray from infinity to the point and counts the number of times the ray crosses the edge of polygon. The crossing number is even if and only if the point is outside the polygon. This algorithm works because if the ray never crosses the polygon, then its crossing number is zero, which is even, and the point is outside. Every time the ray does cross the polygon, the crossing number switches between even and odd, and the point at its tip switches between outside and inside.^{[19]}
Another application comes from the field of graph theory. A graph whose vertices are split into two groups, such that two vertices from the same group are never adjacent, is called a bipartite graph. If a (connected) graph has no odd cycles, then an explicit bipartition can be constructed by choosing a base vertex v and coloring every vertex black or white, depending on whether its distance from v is even or odd. Since the distance between v and itself is 0, and 0 is even, the base vertex is colored the opposite color as its neighbors, which lie at a distance of 1.^{[20]}
Algebraic patterns
The evenness of zero appears in the structured context of abstract algebra. The fact that the additive identity (zero) is even, together with the evenness of sums and additive inverses of even numbers and the associativity of addition, means that the even integers form a group. Moreover, the group of even integers under addition is a subgroup of the group of all integers; this is an elementary example of the subgroup concept.^{[12]} The earlier observation that the rule "even − even = even" forces 0 to be even is part of a general pattern: any nonempty subset of an additive group that is closed under subtraction must be a subgroup, and in particular, must contain the identity.^{[21]}
Since the even integers form a subgroup of the integers, they partition the integers into cosets. These cosets may be described as the equivalence classes of the following equivalence relation: x ~ y if (x − y) is even. Here, the evenness of zero is directly manifested as the reflexivity of the binary relation ~.^{[22]} There are only two cosets of this subgroup—the even and odd numbers—and it can be used as a template for subgroups with index 2 in other groups as well. A wellknown example is the alternating group, a subgroup of the symmetric group on n letters. The elements of the alternating group, called even permutations, are the products of even numbers of transpositions. The identity map, an empty product of no transpositions, is an even permutation since zero is even; it is the identity element of the group.^{[23]}
Adding in the rule "even × integer = even" means that the even numbers form an ideal in the ring of integers, and the above equivalence relation can be described as equivalence modulo this ideal. In particular, even integers are exactly those integers k where k ≡ 0 (mod 2). This formulation is useful for investigating integer zeroes of polynomials.^{[24]}
Degrees of evenness
There is a sense in which some multiples of 2 are "more even" than others. The ancient Greeks already categorized the even numbers as singly and doubly even; 0 is doubly even because it is a multiple of 4, so it can be divided by 2 twice. Indeed, 0 has the unique property of being divisible by every power of 2, so it surpasses all other numbers in "evenness".^{[25]}
One consequence of this fact appears in computer algorithms such as the Cooley–Tukey FFT, in which numbers appear in bitreversed order. This ordering has the property that the farther to the left the first 1 occurs in a number's binary expansion, or the more times it is divisible by 2, the sooner it appears. Zero's bit reversal is still zero; it can be divided by 2 any number of times, and its binary expansion does not contain any 1s, so it always comes first.^{[26]} The illustration on the right depicts the evenness of the integers from +64 to −64.
It is clear that 0 is divisible by 2 more times than any other number, but one runs into trouble when trying to quantify exactly how many times that is. For any nonzero integer n, one may define the 2adic order of n: an integer which can be described as the number of times n is divisible by 2, or the exponent of the largest power of 2 that divides n, or the multiplicity of 2 in the prime factorization of n. But none of these descriptions works for 0; no matter how many times 0 is halved, it can still be halved again. Rather, the usual convention is to set the 2order of 0 to be infinity as a special case.^{[27]} This convention is not peculiar to the 2order; it is one of the axioms of an additive valuation in higher algebra.^{[28]}
The powers of two—1, 2, 4, 8, ...—form a simple sequence of increasingly even numbers. There are mathematically interesting ways to force such sequences to actually converge to zero, including the construction of the 2adic numbers.^{[29]}
History
It is difficult to say when in the history of mathematics the first person examined the parity of zero; there is no welldocumented answer.^{[30]} What is certain is that even and odd numbers were known before the number zero was introduced. Indeed, this historical development parallels children's conceptual development.^{[31]} The algebraic properties of 0, such as the rule 0 × n = 0, were first systematically explored by Indian mathematicians such as Brahmagupta in the 7th century—relatively late in comparison to the early history of number theory.
Ancient Greek mathematicians generally considered 2 to be the first even number and 3 the first odd number, and some did not even recognize 2 as even.^{[32]} The number 1 was not truly a number, but a component of all other numbers; as such it had to be both even and odd, and therefore neither truly even nor truly odd. This dual role for 1 was a source of metaphysical discomfort; one historian asserts that the Greeks could have avoided the issue had they known about 0.^{[33]}
Education
The subject of the parity of zero is often treated within the first two or three years of primary education, as the concept of even and odd numbers is introduced and developed.^{[34]}
Students' knowledge
The chart on the right^{[35]} depicts children's beliefs about the parity of zero, as they progress from Year 1 to Year 6. The data are from Len Frobisher, who conducted a pair of surveys of UK schoolchildren. Frobisher was interested in how knowledge of singledigit parity translates to knowledge of multipledigit parity, and zero figures prominently in the results.^{[36]}
In a preliminary survey of nearly 400 sevenyearolds, 45% chose even over odd when asked the parity of zero.^{[37]} A followup investigation offered more choices: neither, both, and don't know. This time the number of children in the same age range identifying zero as even dropped to 32%.^{[38]} Success in deciding that zero is even initially shoots up and then levels off at around 50% in Years 3 to 6.^{[39]} For comparison, the easiest task, identifying the parity of a single digit, levels off at about 85% success.^{[40]}
In interviews, Frobisher elicited the students' reasoning. One fifthyear decided that 0 was even because it was found on the 2 times table. A couple fourthyears realized that zero can be split into equal parts: "no one gets owt if it's shared out."^{[41]} Another fourthyear reasoned "1 is odd and if I go down it's even."^{[42]} The interviews also revealed the misconceptions behind incorrect responses. A secondyear was "quite convinced" that zero was odd, on the basis that "it is the first number you count".^{[43]} A fourthyear referred to 0 as "none" and thought that it was neither odd nor even, since "it's not a number".^{[44]}
More indepth investigations were conducted by Esther Levenson, Pessia Tsamir, and Dina Tirosh, who interviewed a pair of sixthgrade students who were performing highly in their mathematics class. One student preferred deductive explanations of mathematical claims, while the other preferred practical examples. Both students initially thought that 0 was neither even nor odd, for different reasons. Levenson et al.'s report in the Journal of Mathematical Behavior details the students' reasoning; one of the themes is that their beliefs about the parity of zero are consistent with their concepts of zero and division.^{[45]}
Teachers' knowledge
Researchers of mathematics education at the University of Michigan used the trueorfalse prompt "0 is an even number", among many similar questions, in a 2000–2004 study of 700 primary teachers in the United States. For them the question exemplifies "common knowledge ... that any welleducated adult should have", and it is "ideologically neutral" in that the answer does not vary between traditional and reform mathematics. Overall performance in the study significantly predicted improvements in students' standardized test scores after taking the teachers' classes.^{[46]}
It is uncertain how many teachers harbor misconceptions about zero. The Michigan study did not publish data for individual questions. One report comes from Betty Lichtenberg, who wrote an article titled "Zero is an even number" in the journal The Arithmetic Teacher in 1972. Lichtenberg, an associate professor of mathematics education at the University of South Florida, draws on her experience with a course she and her colleagues taught on methods for teaching arithmetic. She reports that several sections of prospective elementary school teachers were given a trueorfalse test including the item "Zero is an even number." They found it to be a "tricky question", and about two thirds answered "False".^{[47]}
The literature contains a couple data points concerning teachers' attitudes about students' attitudes. The National Council of Teachers of Mathematics's Principles and Standards for School Mathematics records a first grader's argument that zero is an even number: "If zero were odd, then 0 and 1 would be two odd numbers in a row. Even and odd numbers alternate. So 0 must be even..." In a survey of 10 college students preparing to teach mathematics, none of them thought that the argument sufficed as a mathematical proof. When they were told that a first grader had written the argument, most agreed that it was acceptable reasoning for that age level.^{[48]}
Group discussions
Often students will independently ask if zero is even; the Israel National Mathematics Curriculum reminds first grade teachers that zero is even, but advises that it is unnecessary to mention this unless the class brings it up.^{[49]} In one study, Annie Keith observed a class of 15 second grade students, who convinced each other that zero was an even number based on evenodd alternation and on the possibility of splitting a group of zero things in two equal groups.^{[50]}
Deborah Loewenberg Ball, a researcher specializing in mathematics instruction, has extensively analyzed an episode that occurred in her public school classroom of 22 third graders during the 1989–1990 school year. Ball asked the class for comments about even and odd numbers and zero, which they had just been discussing with a group of fourthgraders, and over the next six minutes the students made a variety of claims about the parity of zero, the rules for even numbers, and how mathematics is done. The claims about zero alone take many forms: Zero is not even or odd; Zero could be even; Zero is not odd; Zero has to be an even; Zero is not an even number; Zero is always going to be an even number; Zero is not always going to be an even number; Zero is even; Zero is special.^{[51]} Much of the discussion hinges around one student's comment that "even numbers make even numbers",^{[52]} but that zero is "special"; students seem to advance competing definitions of "even", all the while bringing up examples and counterexamples to each other's claims.^{[53]}
Ball later asked her students to reflect on this "particularly long and confusing discussion on even and odd numbers".^{[54]} One student commented that hearing other ideas had helped her understanding, and she now believed for the first time that zero was even. At the same time, another student had originally thought zero to be even but "got sort of mixed up" and was unsure whom to agree with. Ball finds it significant that the latter student expressed a desire to listen further to the discussion: in this sense, both students have learned something valuable about their own learning process.^{[55]} On another occasion, during a discussion on fractions, Ball asked the class whether voting is a good way to prove what is true in mathematics. One of the students returned to her experience of the discussion on zero, concluding that voting was ineffective in comparison to investigating patterns.^{[56]}
Implications for instruction
Mathematically, no further proof is required, but more explanation is needed in the context of education. One issue concerns the foundations of the proof: the concise definition of "even" given above is not always appropriate. A student in the first years of primary education may not yet have learned what "integer" or "multiple" means, much less how to multiply with 0.^{[57]} Additionally, stating a definition of parity for all integers can seem like an arbitrary conceptual shortcut if the only even numbers investigated so far are 2, 4, 6, 8, and higher. It can help to acknowledge that as the number concept is extended from positive integers to include zero and negative integers, so too number properties such as parity are extended in a nontrivial way.^{[58]}
Ageappropriate explanations that zero is even, then, return to the concrete interpretation of parity in terms of paired objects, or they emphasize the evenodd alternation between numbers. Meanwhile misconceptions about 0 must be combated, such as the belief that 0 means nothing and has no properties.
Numerical cognition
Adults who do believe that zero is even can nevertheless feel unfamiliar or uncomfortable with the fact, enough to measurably slow them down in a reaction time experiment. For an experiment designed to investigate the task of parity determination, a numeral or a number word is flashed to the subject on a monitor, and a computer records the time it takes the subject to identify the number as odd or even by striking an appropriate button, such as a Morse key. Stanislas Dehaene, a pioneer in the field of numerical cognition, led a series of such experiments in the early 1990s. They showed that 0 was slower to process than other even numbers. Some variations of the experiment found delays as long as 60 milliseconds or about 10% of the average reaction time—a small difference but a significant one.^{[60]}
Dehaene's experiments were not designed specifically to investigate 0, but to compare competing models of how parity information is processed and extracted. The most specific extraction model, the mental calculation hypothesis, suggests that reactions to 0 should be fast: 0 is a small number, and it is easy to calculate 0 × 2 = 0. (Subjects are known to compute and name the result of multiplication by zero faster than multiplication of nonzero numbers, although they are slower to verify proposed results like 2 × 0 = 0.) The results of the experiments suggested that something quite different was happening: parity information was apparently being recalled from memory along with a cluster of related properties, such as being prime or a power of two. Both the sequence of powers of two and the sequence of positive evens 2, 4, 6, 8, ... are welldistinguished mental categories whose members are prototypically even. Zero belongs to neither list, hence the slower responses.^{[61]}
Repeated experiments have shown a delay at zero for subjects from a variety of national and linguistic backgrounds, representing both left to right and right to left writing systems; almost all righthanded; from 17–53 years of age; confronted with number names in numeral form, spelled out, and spelled in a mirror image. Dehaene's group did find one differentiating factor: mathematical expertise. In one of their experiments, students in the École Normale Supérieure were divided into two groups: those in literary studies and those studying mathematics, physics, or biology. The slowing at 0 was "essentially found in the [literary] group", and in fact, "before the experiment, some L subjects were unsure whether 0 was odd or even and had to be reminded of the mathematical definition".^{[62]}
This strong dependence on familiarity again undermines the mental calculation hypothesis.^{[63]} The effect also suggests that it is inappropriate to include zero in experiments where even and odd numbers are compared as a group. As one study puts it, "Most researchers seem to agree that zero is not a typical even number and should not be investigated as part of the mental number line."^{[64]}
Everyday contexts
Some of the contexts where the parity of zero makes an appearance are purely rhetorical:
 It provides material for Internet message boards and asktheexpert websites.^{[65]}
 Linguist Joseph Grimes muses that asking "Is zero an even number?" to married couples is a good way to get them to disagree.^{[66]}
 Social theorist Anthony Wilden proposes that all rules have exceptions, giving an example: "Whole numbers are either odd or even, but zero is neither one nor the other."^{[67]}
 Columnist Tony Snow once wrote, "Question: Name one thing beneath Bill Clinton's dignity. Answer: This is a trick question, like asking whether zero is odd or even. It has no known answer."^{[68]} The attack on Clinton was to be expected; three readers who protested the comparison were more concerned with the mathematical error.^{[69]}
 Around the turn of the third millennium, media outlets noted a pair of unusual milestones: "11/19/1999" was the last calendar date composed of all odd digits that would occur for a very long time, and that "02/02/2000" was the first alleven date to occur in a very long time.^{[70]} Since these results make use of 0 being even, some readers disagreed with the idea.^{[71]}
There are also situations where calling zero even, or not, has consequences:
One thirdparty study guide for the GMAT states that 0 is not even,^{[10]} but the test's authors publish an official study guide that explicitly includes 0 in the even numbers. The correct answers to some of the GMAT's "data sufficiency" questions require that the usual rules for even numbers, such as n being even if (n + 2) is even, hold without exception for 0.^{[72]} The GRE also stipulates that 0 is even.^{[73]} Generally, on standardized tests, if a question asks about the behavior of even numbers, it might be necessary to keep in mind that zero is even.^{[74]}
The nominal evenness of zero is relevant to oddeven rationing systems. Cars might be allowed to drive or to purchase gasoline on alternate days, according to the parity of the last digit in their license plates. Half of the numbers in a given range end in 0, 2, 4, 6, 8 and the other half in 1, 3, 5, 7, 9, so it makes sense to include 0 with the other even numbers. The relevant legislation sometimes stipulates that zero is even to avoid confusion.^{[75]} In fact, an oddeven restriction on driving in 1977 Paris did lead to confusion when the rules were unclear. On an oddonly day, the police avoided fining drivers whose plates ended in 0, because they did not know whether 0 was even.^{[76]}
In other situations, it can make sense to separate 0 from the other even numbers. On U.S. Navy vessels, evennumbered compartments are found on the port side, but zero is reserved for compartments that intersect the centerline. That is, the numbers read ...6420135... from port to starboard.^{[77]}
In the game of roulette, the casino has an interest in making sure that less than half of the numbers are counted as even. Thus the number 0 does not count as even or odd; a bet placed on either even or odd does not win if the ball falls on "0" or "00". The exact result depends on local rules, but the overall effect is to give the house an edge on "even money" bets.^{[78]} Similarly, the parity of zero can affect payoffs in prop bets when the outcome depends on whether some randomized number is odd or even, and it turns out to be zero.^{[79]} One bookmaker offers a "cricket roulette" in which a batsman who is dismissed for a duck wins for the bank.^{[80]}
The game of "odds and evens" is also affected: if both players cast zero fingers, who wins? Generally zero is counted as even.^{[81]} In fact, playing this game has been suggested as a way of introducing children to the concept that 0 is divisible by 2.^{[82]}
References
 ^ Penner 1999, p. 34: Lemma B.2.2, The integer 0 is even and is not odd. Penner uses the mathematical symbol ∃, the existential quantifier, to state the proof: "To see that 0 is even, we must prove that ∃k(0 = 2k), and this follows from the equality 0 = 2 · 0."
 ^ Ball, Lewis & Thames (2008, p. 15) discuss this challenge for the elementarygrades teacher, who wants to give mathematical reasons for mathematical facts, but whose students neither use the same definition, nor would understand it if it were introduced.
 ^ Compare Lichtenberg (1972, p. 535) Fig. 1
 ^ Lichtenberg 1972, pp. 535–536 "...numbers answer the question How many? for the set of objects ... zero is the number property of the empty set ... If the elements of each set are marked off in groups of two ... then the number of that set is an even number."
 ^ Lichtenberg 1972, pp. 535–536 "Zero groups of two stars are circled. No stars are left. Therefore, zero is an even number."
 ^ Lichtenberg 1972, p. 537; compare her Fig. 3. "If the even numbers are identified in some special way ... there is no reason at all to omit zero from the pattern."
 ^ Lichtenberg 1972, pp. 537–538 "At a more advanced level ... numbers expressed as (2 × ▢) + 0 are even numbers ... zero fits nicely into this pattern.
 ^ Gowers 2002, p. 118 "The seemingly arbitrary exclusion of 1 from the definition of a prime … does not express some deep fact about numbers: it just happens to be a useful convention, adopted so there is only one way of factorizing any given number into primes."
 ^ ^{a} ^{b} ^{c} Partee 1978, p. xxi
 ^ ^{a} ^{b} ^{c} Stewart 2001, p. 54 These rules are given, but they are not quoted verbatim.
 ^ Devlin 1985, pp. 30–33
 ^ ^{a} ^{b} Berlinghoff, Grant & Skrien 2001 For isolated vertices see p.149; for groups see p.311.
 ^ Lovász, Pelikán & Vesztergombi 2003, pp. 127–128
 ^ Starr 1997, pp. 58–62
 ^ Border 1985, pp. 23–25
 ^ Lorentz 1994, pp. 5–6; Nipkow, Paulson & Wenzel 2002, p. 127
 ^ Bunch 1982, p. 165
 ^ Salzmann et al. 2007, p. 168
 ^ Wise 2002, pp. 66–67
 ^ Anderson 2001, p. 53; Hartsfield & Ringel 2003, p. 28
 ^ Dummit & Foote 1999, p. 48
 ^ Andrews 1990, p. 100
 ^ Tabachnikova & Smith 2000, p. 99; Anderson & Feil 2005, pp. 437–438
 ^ Barbeau 2003, p. 98
 ^ Arnold 1919, p. 21 "By the same test zero surpasses all numbers in 'evenness.'"; Wong 1997, p. 479 "Thus, the integer b000⋯000 = 0 is the most 'even.'
 ^ Wong 1997, p. 479
 ^ Gouvêa 1997, p. 25 Of a general prime p: "The reasoning here is that we can certainly divide 0 by p, and the answer is 0, which we can divide by p, and the answer is 0, which we can divide by p…" (ellipsis in original)
 ^ Krantz 2001, p. 4
 ^ Salzmann et al. 2007, p. 224
 ^ Indeed, it is difficult to find any reliable information on the history of the parity of zero. Historians writing about parity tend to mention only ancient Greek and Chinese mathematicians, and historians writing about zero tend not to mention parity. According to two modern writers, the recognition of zero as an even number followed soon after its invention. (These authors do not cite sources for their claims, so they are difficult to corroborate.) Cencini (2003, p. 299), an economics book, attributes the first statement to Indian mathematicians in the time of Brahmagupta. Haven (1998, p. 13), a work of historical fiction, suggests that alKhwārizmī became the first to call 0 even during his arguments to the Caliph that sifr was a number. Haven (2005, p. 28) references himself in making the same claim. In more modern times, a claim that zero is even appears in Chase (1849, p. 65).
 ^ Levenson, Tsamir & Tirosh 2007, p. 84
 ^ Plato & Allen 1997, pp. 262–264
 ^ Guthrie 1992, pp. 239–242
 ^ This is the timeframe in United States, Canada, Great Britain, Australia, and Israel; see Levenson, Tsamir & Tirosh (2007, p. 85).
 ^ ^{a} ^{b} Frobisher 1999, p. 41
 ^ Frobisher 1999, pp. 31 (Introduction); 40–41 (The number zero); 48 (Implications for teaching)
 ^ Frobisher 1999, pp. 37, 40, 42; results are from the survey conducted in the midsummer term of 1992.
 ^ Frobisher 1999, p. 41 "The percentage of Year 2 children deciding that zero is an even number is much lower than in the previous study, 32 per cent as opposed to 45 per cent"
 ^ Frobisher 1999, p. 41 "The success in deciding that zero is an even number did not continue to rise with age, with approximately one in two children in each of Years 2 to 6 putting a tick in the 'evens' box ..."
 ^ Frobisher 1999, pp. 40–42, 47; these results are from the February 1999 study, including 481 children, from three schools at a variety of attainment levels.
 ^ Frobisher 1999, p. 41, attibuted to "Ben"
 ^ Frobisher 1999, p. 41, attributed to "Jonathan"
 ^ Frobisher 1999, p. 41, attributed to "Joseph"
 ^ Frobisher 1999, p. 41, attributed to "Richard"
 ^ Levenson, Tsamir & Tirosh 2007, pp. 83–95
 ^ Ball, Hill & Bass 2005, pp. 14–16
 ^ Lichtenberg 1972, p. 535
 ^ Dickerson 2006, p. 712
 ^ Levenson, Tsamir & Tirosh 2007, p. 86, referring to the 2005 INMC
 ^ Keith 2006, pp. 35–68 "There was little disagreement on the idea of zero being an even number. The students convinced the few who were not sure with two arguments. The first argument was that numbers go in a pattern ...odd, even, odd, even, odd, even... and since two is even and one is odd then the number before one, that is not a fraction, would be zero. So zero would need to be even. The second argument was that if a person has zero things and they put them into two equal groups then there would be zero in each group. The two groups would have the same amount, zero"
 ^ Ball, Lewis & Thames 2008, p. 27, Figure 1.5 "Mathematical claims about zero."
 ^ Ball, Lewis & Thames 2008, p. 34, attributed to "Nathan"
 ^ Ball, Lewis & Thames 2008, 13–44 and 195–200
 ^ Ball 1993, p. 392. This quotation is from the journal article; those words were not necessarily used in the class discussion.
 ^ Ball 1993, pp. 373–397 Quotation on p.392; emphasis is the author's.
 ^ Ball 1992, pp. 12–13 Betsy: I have an example of why voting doesn't work because when we were talking about zero, if it was an odd or even. A whole lot of people said that it was an odd but then afterwards we figured out that it was even and voting didn't help us know if it was odd or even because the answer was opposite than what people had voted. Teacher: So how did we change our minds then if the voting doesn't work? Betsy: Because the people found out patterns and the number line and they figured out that no, zero must not be a odd because when it goes up there it goes odd, even, odd, even, odd, even and so when you had an odd number like one and then you have zero, zero must be even because that's the way it is.
 ^ Ball, Lewis & Thames 2008, p. 15. See also Ball's keynote for further discussion of appropriate definitions.
 ^ As concluded by Levenson, Tsamir & Tirosh (2007, p. 93), referencing Freudenthal (1983, p. 460)
 ^ The following quote from Nuerk, Iversen & Willmes (2004, p. 851) is modified with square brackets in order to fit the colored graphic: "SSA for positive ... Arabic numerals 0 to 8 responded to with the right hand [green and pink] and the left hand [blue and orange]. MARCcompatible conditions [green and blue] and MARCincompatible conditions [pink and orange] are depicted differently. Again, parity, magnitude, and response hand did not lead to separate grouping of different conditions, but only MARC compatibility or incompatibility. It can also be seen that zero strongly differs from all other numbers regardless of whether it is responded to with the left or the right hand. (See the line that separates zero from the other numbers.)"
 ^ See data throughout Dehaene, Bossini & Giraux (1993), and summary by Nuerk, Iversen & Willmes (2004, p. 837).
 ^ Dehaene, Bossini & Giraux 1993, pp. 374–376
 ^ Dehaene, Bossini & Giraux 1993, pp. 376–377
 ^ Dehaene, Bossini & Giraux 1993, p. 376 "In some intuitive sense, the notion of parity is familiar only for numbers larger than 2. Indeed, before the experiment, some L subjects were unsure whether 0 was odd or even and had to be reminded of the mathematical definition. The evidence, in brief, suggests that instead of being calculated on the fly by using a criterion of divisibility by 2, parity information is retrieved from memory together with a number of other semantic properties ... If a semantic memory is accessed in parity judgments, then interindividual differences should be found depending on the familiarity of the subjects with number concepts."
 ^ Nuerk, Iversen & Willmes 2004, pp. 838, 860–861
 ^ The Math Forum participants 2000; Straight Dope Science Advisory Board 1999; Doctor Rick 2001
 ^ Grimes 1975, p. 156 "...one can pose the following questions to married couples of his acquaintance: (1) Is zero an even number? ... Many couples disagree..."
 ^ Wilden & Hammer 1987, p. 104
 ^ Snow 2001
 ^ Parker 2001 "Finally, give that poor imagination a rest and be assured that ... zero is an even number."; Wilson 2001 "Even my 9yearold granddaughter knows what Snow doesn't, that zero is an even number."; Morgan 2001 "Questionable Mathematics ... Zero is even of course."
 ^ Steinberg 1999; Siegel 1999; Stingl 2006
 ^ Sones & Sones 2002 "It follows that zero is even, and that 2/20/2000 nicely cracks the puzzle. Yet it's always surprising how much people are bothered by calling zero even..."; Column 8 readers 2006a "'...according to mathematicians, the number zero, along with negative numbers and fractions, is neither even nor odd,' writes Etan..."; Column 8 readers 2006b "'I agree that zero is even, but is Professor Bunder wise to 'prove' it by stating that 0 = 2 x 0? By that logic (from a PhD in mathematical logic, no less), as 0 = 1 x 0, it's also odd!' The prof will dispute this and, logically, he has a sound basis for doing so, but we may be wearing this topic a little thin ..."
 ^ Graduate Management Admission Council 2005, pp. 108, 295–297
 ^ Educational Testing Service 2009, p. 1
 ^ Kaplan Staff 2004, p. 227
 ^ Sones & Sones 2002 "Penn State mathematician George Andrews, who recalls a time of gas rationing in Australia ... Then someone in the New South Wales parliament asserted this meant plates ending in zero could never get gas, because 'zero is neither odd nor even. So the New South Wales parliament ruled that for purposes of gas rationing, zero is an even number!'" In another example, a 1980 Maryland law specifies, "(a) On even numbered calendar dates gasoline shall only be purchased by operators of vehicles bearing personalized registration plates containing no numbers and registration plates with the last digit ending in an even number. This shall not include ham radio operator plates. Zero is an even number; (b) On odd numbered calendar dates ..." Partial quotation taken from Google book search, accessed on 20080222.
 ^ Arsham 2002; The quote is attributed to the heute broadcast of October 1, 1977. Arsham's account is repeated by Crumpacker (2007, p. 165).
 ^ Cutler 2008, pp. 237–238
 ^ Brisman 2004, p. 153
 ^ Smock 2006; Hohmann 2007
 ^ Turner 1996
 ^ Diagram Group 1983, p. 213
 ^ Baroody & Coslick 1998, p. 1.33
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 Further reading
 Ball, Deborah Loewenberg (1989), Breaking with experience in learning to teach mathematics: the role of a preservice methods course, East Lansing, MI: National Center for Research on Teacher Education
 Ball, Deborah Loewenberg (1997), "What do students know? Facing challenges of distance, context, and desire in trying to hear children", in B. Biddle, T. Good, & I. Goodson, International handbook on teachers and teaching, 2, Dordrecht, Netherlands: Kluwer Press, pp. 679–718, http://wwwpersonal.umich.edu/~dball/chapters/BallWhatDoStudentsKnow.pdf, retrieved 23 August 2009
 Ball, Deborah Loewenberg; Bass, Hyman (2000), "Interweaving content and pedagogy in teaching and learning to teach: Knowing and using mathematics", in J. Boaler, Multiple perspectives on the teaching and learning of mathematics, Westport, CT: Ablex, pp. 83–104, http://wwwpersonal.umich.edu/~dball/chapters/BallBassInterweavingContent.pdf, retrieved 23 August 2009
 Ball, Deborah Loewenberg; Bass, Hyman (2000), "Making believe: The collective construction of public mathematical knowledge in the elementary classroom", in D. Phillips, Yearbook of the National Society for the Study of Education, Constructivism in Education, Chicago: University of Chicago Press, pp. 193–224, http://wwwpersonal.umich.edu/~dball/chapters/BallBassMakingBelieve.pdf, retrieved 23 August 2009
 Barnett, S. (1990), Matrices: methods and applications, Oxford Applied Mathematics and Computing Science Series, p. 115
 Cameron, Peter Jephson (1994), Combinatorics: topics, techniques, algorithms, p. 167
 Dulski, Thomas R. (1999), Trace elemental analysis of metals: methods and techniques, p. 549
 Evans, James Robert; Minieka, Edward (1992), Optimization algorithms for networks and graphs (2nd ed.), p. 283
 Fias, Wim (November 2001), "Two routes for the processing of verbal numbers: evidence from the SNARC effect", Psychological Research 65 (4): 250–259, doi:10.1007/s004260100065, PMID 11789429
 Golomb, Solomon Wolf (1994), Polyominoes: Puzzles, Patterns, Problems, and Packings, Princeton: Princeton University Press, p. 119, ISBN 0691024448
 Kalsi, P. S. (2004), Spectroscopy of Organic Compounds (6th ed.), New Age International Publishers, p. 437
 Koshy, Thomas (2004), Discrete mathematics with applications, Elsevier, p. 765
 Leinhardt, Gaea; Steele, Michael D. (2005), "Seeing the Complexity of Standing to the Side: Instructional Dialogues", Cognition and Instruction (Lawrence Erlbaum) 23 (1): 87–163, doi:10.1207/s1532690xci2301_4, ERIC # EJ724916
 Lengyel, Tamas (1994), "Characterizing the 2adic order of the logarithm", The Fibonacci Quarterly 32: 397–401, http://employees.oxy.edu/lengyel/papers/FQ/pdf/log2.pdf, retrieved 23 August 2009
 Merttens, Ruth; Vass, Jeff (1990), Sharing maths cultures: IMPACT, Inventing Maths for Parents and Children and Teachers, The Falmer Press, p. 70
 Peterson, Penelope L. (1994), "Research Studies as Texts: Sites for Exploring the Beliefs and Learning of Researchers and Teachers", in Ruth Garner and Patricia A. Alexander, Beliefs about Text and Instruction with Text (1st ed.), pp. 93–120
 Schoenfeld, Alan H. (2002), "A highly interactive discourse structure", in J. Brophy, Social constructivist teaching: Its affordances and constraints, Advances in Research on Teaching, 9, Elsevier, doi:10.1016/S14793687(02)800074, ISBN 0762308737, http://gse.berkeley.edu/faculty/AHSchoenfeld/Schoenfeld_Discourse.pdf, retrieved 23 August 2009
 Stirling, David S. G. (1997), Mathematical analysis and proof, Albion Mathematics & Applications Series (1st ed.), p. 225
 United States Senate Committee on Energy and Natural Resources, Subcommittee on Energy Conservation and Supply (1980), Energy Management Partnership Act of 1979: hearing before the Subcommittee on Energy Conservation and Supply of the Committee on Energy and Natural Resources, United States Senate, Ninetysixth Congress, first session on S. 1280, Washington, D.C.: U.S. G.P.O., LCCN 80602009
External links
 Doctor Rick (2001), "Is Zero Even?", Ask Dr. Math (The Math Forum), http://mathforum.org/library/drmath/view/57188.html, retrieved 20070924
 Straight Dope Science Advisory Board (1999), "Is zero odd or even?", The Straight Dope Mailbag, http://www.straightdope.com/mailbag/mzeroeven.html, retrieved 20070924
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