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 A217869 a(n) is the sum of total number of positive integer solutions to each of a^2 + b^2 = n, a^2 + 2*b^2 = n, a^2 + 3*b^2 = n and a^2 + 7*b^2 = n. (Order matters for the equation a^2+b^2 = n). 1
 0, 1, 1, 1, 2, 1, 1, 2, 1, 2, 2, 2, 3, 0, 0, 2, 3, 2, 2, 2, 1, 1, 1, 1, 2, 2, 2, 3, 3, 0, 1, 3, 2, 3, 0, 2, 4, 1, 1, 2, 3, 0, 3, 2, 2, 0, 0, 2, 1, 3, 2, 5, 3, 2, 0, 1, 3, 2, 1, 0, 3, 0, 1, 3, 4, 2, 3, 3, 0, 0, 1, 3, 4, 2, 1, 4, 1, 0, 2, 2, 2, 3, 1, 3, 4, 1, 0, 3, 3, 2, 2, 1, 1, 0, 0, 1, 4, 1, 4, 3 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,5 COMMENTS Note: For the equation a^2 + b^2 = n, if there are two solutions (a,b) and (b,a), then they will be counted separately. The sequences A216501 and A216671 give how many of the four k values, k = 1, 2, 3, 7 does the equation a^2 + k*b^2 = n have a solution to. 1, 2, 3, 7 are the first four numbers, with the class number 1. a(n) = A217463(n), when n is not the sum of two positive squares. But when n is the sum of two positive squares, the ordered pairs for the equation x^2+y^2 = n count. For example, 193 = 12^2 + 7^2. 193 = 7^2 + 12^2. 193 = 11^2 + 2*6^2. 193 = 1^2 + 3*8^2. 193 = 9^2 + 7*4^2. So, a(193) = 5. On the other hand, for the sequence A217463, the ordered pairs 12^2 + 7^2, 7^2 + 12^2 will be counted only once, so A217463(193) = 4. REFERENCES David A. Cox, Primes of the Form x^2 + n y^2, Wiley, 1989. LINKS Table of n, a(n) for n=1..100. PROG (PARI) for(n=1, 100, sol=0; for(x=1, 100, if(issquare(n-x*x)&&n-x*x>0, sol++); if(issquare(n-2*x*x)&&n-2*x*x>0, sol++); if(issquare(n-3*x*x)&&n-3*x*x>0, sol++); if(issquare(n-7*x*x)&&n-7*x*x>0, sol++)); printf(sol", ")) CROSSREFS Cf. A216501, A216671. Cf. A217463 (related sequence of this when the order does not matter for the equation a^2 + b^2 = n). Cf. A216501 (how many of the four k values, k = 1, 2, 3, 7 does the equation a^2 + k*b^2 = n have a solution to, with a > 0, b > 0). Cf. A216671 (how many of the four k values, k = 1, 2, 3, 7 does the equation a^2 + k*b^2 = n have a solution to, with a >= 0, b >= 0). Cf. A063725 (number of solutions to n = a^2+b^2 (when the solutions (a, b) and (b, a) are being counted differently) with a > 0, b > 0). Cf. A216278 (number of solutions to n = a^2+2*b^2 with a > 0, b > 0). Cf. A092573 (number of solutions to n = a^2+3*b^2 with a > 0, b > 0). Cf. A216511 (number of solutions to n = a^2+7*b^2 with a > 0, b > 0). Sequence in context: A281527 A124038 A029311 * A116674 A025836 A029319 Adjacent sequences: A217866 A217867 A217868 * A217870 A217871 A217872 KEYWORD nonn AUTHOR V. Raman, Oct 13 2012 STATUS approved

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Last modified May 22 12:56 EDT 2024. Contains 372755 sequences. (Running on oeis4.)