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  2. Euler's totient function - Wikipedia

    en.wikipedia.org/wiki/Euler's_totient_function

    If n is a power of an odd prime number the formula for the totient says its totient can be a power of two only if n is a first power and n − 1 is a power of 2. The primes that are one more than a power of 2 are called Fermat primes, and only five are known: 3, 5, 17, 257, and 65537. Fermat and Gauss knew of these.

  3. Double factorial - Wikipedia

    en.wikipedia.org/wiki/Double_factorial

    In mathematics, the double factorial of a number n, denoted by n‼, is the product of all the positive integers up to n that have the same parity (odd or even) as n. [1] That is, n ! ! = ∏ k = 0 ⌈ n 2 ⌉ − 1 ( n − 2 k ) = n ( n − 2 ) ( n − 4 ) ⋯ . {\displaystyle n!!=\prod _{k=0}^{\left\lceil {\frac {n}{2}}\right\rceil -1}(n-2k ...

  4. Factorial - Wikipedia

    en.wikipedia.org/wiki/Factorial

    These numbers form the discriminants of Hermite polynomials. [100] They can be continuously interpolated by the K-function, [101] and obey analogues to Stirling's formula [102] and Wilson's theorem. [103] Jordan–Pólya numbers The Jordan–Pólya numbers are the products of factorials, allowing

  5. Even and odd functions - Wikipedia

    en.wikipedia.org/wiki/Even_and_odd_functions

    The product of two even functions is an even function. That implies that product of any number of even functions is an even function as well. The product of two odd functions is an even function. The product of an even function and an odd function is an odd function. The quotient of two even functions is an even function.

  6. Formula for primes - Wikipedia

    en.wikipedia.org/wiki/Formula_for_primes

    In number theory, a formula for primes is a ... the first factor in the product becomes one, and the formula produces ... (n + 1, a n) are always odd and so never ...

  7. Euclid's theorem - Wikipedia

    en.wikipedia.org/wiki/Euclid's_theorem

    5.2 Proof using Legendre's formula. ... Let P be the product of all the prime numbers in the list: ... is an odd number in the set , so =, or = ...

  8. Pentagonal number theorem - Wikipedia

    en.wikipedia.org/wiki/Pentagonal_number_theorem

    In summary, it has been shown that partitions into an even number of distinct parts and an odd number of distinct parts exactly cancel each other, producing null terms 0x n, except if n is a generalized pentagonal number = = /, in which case there is exactly one Ferrers diagram left over, producing a term (−1) k x n. But this is precisely ...

  9. Square number - Wikipedia

    en.wikipedia.org/wiki/Square_number

    This is also equal to the sum of the first n odd numbers as can be seen in the above pictures, where a square results from the previous one by adding an odd number of points (shown in magenta). The formula follows: = = (). For example, 5 2 = 25 = 1 + 3 + 5 + 7 + 9.