Vampire number

Vampire number

In mathematics, a vampire number (or true vampire number) is a composite natural number "v", with an even number of digits "n", that can be factored into two integers "x" and "y" each with "n"/2 digits and not both with trailing zeroes, where "v" contains all the digits from "x" and from "y", in any order. "x" and "y" are called the fangs.

For example: 1260 is a vampire number, with 21 and 60 as fangs, since 21 × 60 = 1260. However, 126000 (which can be expressed as 210 × 600) is not, as both 210 and 600 have trailing zeroes.

Vampire numbers first appeared in a 1994 post by Clifford A. Pickover to the Usenet group sci.math, and the article he later wrote was published in chapter 30 of his book "Keys to Infinity".

The vampire numbers are:

1260, 1395, 1435, 1530, 1827, 2187, 6880, 102510, 104260, 105210, 105264, 105750, 108135, 110758, 115672, 116725, 117067, 118440, 120600, 123354, 124483, 125248, 125433, 125460, 125500, ... OEIS|id=A014575

There are many known sequences of infinitely many vampire numbers following a pattern, such as:: 1530 = 30×51, 150300 = 300×501, 15003000 = 3000×5001, ...

Multiple fang pairs

A vampire number can have multiple distinct pairs of fangs. The first of infinitely many vampire numbers with 2 pairs of fangs:

:125460 = 204 × 615 = 246 × 510

The first with 3 pairs of fangs::13078260 = 1620 × 8073 = 1863 × 7020 = 2070 × 6318

The first with 4 pairs of fangs::16758243290880 = 1982736 × 8452080 = 2123856 × 7890480 = 2751840 × 6089832 = 2817360 × 5948208

The first with 5 pairs of fangs::24959017348650 = 2947050 × 8469153 = 2949705 × 8461530 = 4125870 × 6049395 = 4129587 × 6043950 = 4230765 × 5899410

Variants

Pseudovampire numbers are similar to vampire numbers, except that the fangs of an "n"-digit pseudovampire number need not be of length "n"/2 digits. Pseudovampire numbers can have an odd number of digits, for example 126 = 6×21.

More generally, you can allow more than two fangs. In this case, vampire numbers are numbers "n" which can be factorized using the digits of "n". For example, 1395 = 5×9×31. This sequence starts OEIS|id=A020342::126, 153, 688, 1206, 1255, 1260, 1395, ...

A prime vampire number, as defined by Carlos Rivera in 2002, is a true vampire number whose fangs are its prime factors. The first few prime vampire numbers are::117067, 124483, 146137, 371893, 536539

As of 2006 the largest known is the square (94892254795×1045418+1)2, found by Jens K. Andersen in 2002.

References

* Pickover, Clifford A. (1995). "Keys to Infinity". Wiley. ISBN 0-471-19334-8
* [http://groups.google.com/group/sci.math/msg/f17b2281a4aa16da?lr=&ie=UTF-8 Pickover's original post describing vampire numbers]
* Andersen, Jens K. [http://hjem.get2net.dk/jka/math/vampires/ "Vampire Numbers"]
* Rivera, Carlos. [http://www.primepuzzles.net/puzzles/puzz_199.htm "The Prime-Vampire numbers"]

External links

*
* Schneider, Walter. [http://web.archive.org/web/20060504235612/http://wschnei.de/digit-related-numbers/vampire.html "Vampire Numbers"]


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