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  2. Jacobi's four-square theorem - Wikipedia

    en.wikipedia.org/wiki/Jacobi's_four-square_theorem

    The number of ways to represent n as the sum of four squares is eight times the sum of the divisors of n if n is odd and 24 times the sum of the odd divisors of n if n is even (see divisor function), i.e.

  3. Hurwitz's theorem (composition algebras) - Wikipedia

    en.wikipedia.org/wiki/Hurwitz's_theorem...

    Since the sum of the squares of the dimensions equals |G| and the dimensions divide |G|, the two irreducibles must have dimension 2 (N − 2)/2. When N is even, there are two and their dimension must divide the order of the group, so is a power of two, so they must both have dimension 2 (N − 2)/2. The space on which the V i 's act can be ...

  4. Hilbert's seventeenth problem - Wikipedia

    en.wikipedia.org/wiki/Hilbert's_seventeenth_problem

    A result of Albrecht Pfister [8] shows that a positive semidefinite form in n variables can be expressed as a sum of 2 n squares. [9] Dubois showed in 1967 that the answer is negative in general for ordered fields. [10] In this case one can say that a positive polynomial is a sum of weighted squares of rational functions with positive ...

  5. Lagrange's four-square theorem - Wikipedia

    en.wikipedia.org/wiki/Lagrange's_four-square_theorem

    The number of representations of a natural number n as the sum of four squares of integers is denoted by r 4 (n). Jacobi's four-square theorem states that this is eight times the sum of the divisors of n if n is odd and 24 times the sum of the odd divisors of n if n is even (see divisor function), i.e.

  6. Waring's problem - Wikipedia

    en.wikipedia.org/wiki/Waring's_problem

    G(3) is at least 4 (since cubes are congruent to 0, 1 or −1 mod 9); for numbers less than 1.3 × 10 9, 1 290 740 is the last to require 6 cubes, and the number of numbers between N and 2N requiring 5 cubes drops off with increasing N at sufficient speed to have people believe that G(3) = 4; [17] the largest number now known not to be a sum of ...

  7. 4 - Wikipedia

    en.wikipedia.org/wiki/4

    Lagrange's four-square theorem states that every positive integer can be written as the sum of at most four squares. [5] [6] Four is one of four all-Harshad numbers. Each natural number divisible by 4 is a difference of squares of two natural numbers, i.e. =. A four-sided plane figure is a quadrilateral or quadrangle, sometimes also called a ...

  8. NYT ‘Connections’ Hints and Answers Today, Friday, December 13

    www.aol.com/nyt-connections-hints-answers-today...

    Spoilers ahead! We've warned you. We mean it. Read no further until you really want some clues or you've completely given up and want the answers ASAP. Get ready for all of today's NYT ...

  9. Sum of squares function - Wikipedia

    en.wikipedia.org/wiki/Sum_of_squares_function

    The number of ways to write a natural number as sum of two squares is given by r 2 (n).It is given explicitly by = (() ())where d 1 (n) is the number of divisors of n which are congruent to 1 modulo 4 and d 3 (n) is the number of divisors of n which are congruent to 3 modulo 4.