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  2. Fermat's theorem on sums of two squares - Wikipedia

    en.wikipedia.org/wiki/Fermat's_theorem_on_sums_of...

    For the avoidance of ambiguity, zero will always be a valid possible constituent of "sums of two squares", so for example every square of an integer is trivially expressible as the sum of two squares by setting one of them to be zero. 1. The product of two numbers, each of which is a sum of two squares, is itself a sum of two squares.

  3. Digit sum - Wikipedia

    en.wikipedia.org/wiki/Digit_sum

    The concept of a decimal digit sum is closely related to, but not the same as, the digital root, which is the result of repeatedly applying the digit sum operation until the remaining value is only a single digit. The decimal digital root of any non-zero integer will be a number in the range 1 to 9, whereas the digit sum can take any value.

  4. Double factorial - Wikipedia

    en.wikipedia.org/wiki/Double_factorial

    The symbolical representation of the results of this paper is much facilitated by the introduction of a separate symbol for the product of alternate factors, , if be odd, or if be odd [sic]. I propose to write n ! ! {\displaystyle n!!} for such products, and if a name be required for the product to call it the "alternate factorial" or the ...

  5. Factorial - Wikipedia

    en.wikipedia.org/wiki/Factorial

    Here () denotes the sum of the base-digits of , and the exponent given by this formula can also be interpreted in advanced mathematics as the p-adic valuation of the factorial. [54] Applying Legendre's formula to the product formula for binomial coefficients produces Kummer's theorem , a similar result on the exponent of each prime in the ...

  6. Summation - Wikipedia

    en.wikipedia.org/wiki/Summation

    In mathematics, summation is the addition of a sequence of numbers, called addends or summands; the result is their sum or total.Beside numbers, other types of values can be summed as well: functions, vectors, matrices, polynomials and, in general, elements of any type of mathematical objects on which an operation denoted "+" is defined.

  7. Composite number - Wikipedia

    en.wikipedia.org/wiki/Composite_number

    One way to classify composite numbers is by counting the number of prime factors. A composite number with two prime factors is a semiprime or 2-almost prime (the factors need not be distinct, hence squares of primes are included). A composite number with three distinct prime factors is a sphenic number. In some applications, it is necessary to ...

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  9. Fermat number - Wikipedia

    en.wikipedia.org/wiki/Fermat_number

    Yves Gallot's proth.exe has been used to find factors of large Fermat numbers. Édouard Lucas , improving Euler's above-mentioned result, proved in 1878 that every factor of the Fermat number F n {\displaystyle F_{n}} , with n at least 2, is of the form k × 2 n + 2 + 1 {\displaystyle k\times 2^{n+2}+1} (see Proth number ), where k is a ...