Perfect power

In mathematics, a perfect power is a number that can be expressed as a power of some whole number above 1. More formally, "n" is a perfect power if there exist natural numbers "m" > 1, and "k" > 1 such that "m^k" = "n". In this case, "n" may be called a perfect "k"th power. If "k" = 2 or "k" = 3, then "n" is called a perfect square or perfect cube, respectively. Sometimes 1 is also considered a perfect power (1^"k" = 1 for any "k").

Examples and sums

A sequence of perfect powers can be generated by iterating through the possible values for "m" and "k". The first few ascending perfect powers in numerical order (showing duplicate powers) are OEIS|id=A072103: :$2^2\; =\; 4,\; 2^3\; =\; 8,\; 3^2\; =\; 9,\; 2^4\; =\; 16,\; 4^2\; =\; 16,\; 5^2\; =\; 25,\; 3^3\; =\; 27,$ $2^5\; =\; 32,\; 6^2\; =\; 36,\; 7^2\; =\; 49,\; 2^6\; =\; 64,\; 4^3\; =\; 64,\; 8^2\; =\; 64,\; dots$

The sum of their reciprocals (including duplicates) is 1::$sum\_\{m=2\}^\{infty\}\; sum\_\{k=2\}^\{infty\}frac\{1\}\{m^k\}=1.$

The first perfect powers without duplicates are (OEIS2C|id=A001597)::(sometimes 1), 4, 8, 9, 16, 25, 27, 32, 36, 49, 64, 81, 100, 121, 125, 128, 144, 169, 196, 216, 225, 243, 256, 289, 324, 343, 361, 400, 441, 484, ...

According to Euler, Goldbach showed (in a now lost letter) that the sum of 1/("p"−1) over the set of perfect powers "p", excluding 1 and excluding duplicates, is 1:

:$sum\_\{p\}frac\{1\}\{p-1\}=\; \{frac\{1\}\{3\}\; +\; frac\{1\}\{7\}\; +\; frac\{1\}\{8\}+\; frac\{1\}\{15\}\; +\; frac\{1\}\{24\}\; +\; frac\{1\}\{26\}+\; frac\{1\}\{31+\; cdots\; =\; 1.$

This is sometimes known as the Goldbach-Euler theorem.

Detecting perfect powers

Detecting whether or not a given natural number "n" is a perfect power may be accomplished in many different ways, with varying levels of complexity. One of the simplest such methods is to consider all possible values for "k" across each of the divisors of "n", up to $k\; leq\; log\_2\; n$. So if the factors of $n$ are $n\_1cdot\; n\_2cdot\; dots\; n\_j$ then one of the values $n\_1^2,\; n\_2^2,\; dots,\; n\_j^2,\; n\_1^3,\; n\_2^3,\; dots$ must be equal to "n" if "n" is indeed a perfect power.

This method can immediately be simplified by instead considering only prime values of "k". This is because if $n\; =\; m^k$ for a composite $k\; =\; ap$ where "p" is prime, then this can simply be rewritten as $n\; =\; m^k\; =\; m^\{ap\}\; =\; (m^a)^p$. Because of this result, the minimal value of "k" must necessarily be prime.

ee also

* Mihăilescu's theorem

References

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External links

* [http://www.recercat.net/bitstream/2072/920/1/776.pdf On a series of Goldbach and Euler]
*MathWorld|urlname=PerfectPower|title=Perfect Power