A217868 a(n) is the sum of total number of nonnegative 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).

5, 2, 2, 6, 2, 1, 2, 3, 6, 2, 2, 3, 3, 0, 0, 7, 3, 3, 2, 2, 1, 1, 1, 1, 7, 2, 3, 4, 3, 0, 1, 4, 2, 3, 0, 7, 4, 1, 1, 2, 3, 0, 3, 2, 2, 0, 0, 3, 6, 4, 2, 5, 3, 2, 0, 1, 3, 2, 1, 0, 3, 0, 2, 8, 4, 2, 3, 3, 0, 0, 1, 4, 4, 2, 2, 4, 1, 0, 2, 2, 7, 3, 1, 3, 4, 1, 0, 3, 3, 2, 2, 1, 1, 0, 0, 1, 4, 2, 4, 8

Offset

1,1

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 class number 1.

a(n) = A217462(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 A217462, the ordered pairs 12^2 + 7^2, 7^2 + 12^2 will be counted only once, so A217462(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=0, 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. A217462 (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. A000925 (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. A216282 (number of solutions to n = a^2+2*b^2 with a >= 0, b >= 0).

Cf. A119395 (number of solutions to n = a^2+3*b^2 with a >= 0, b >= 0).

Cf. A216512 (number of solutions to n = a^2+7*b^2 with a >= 0, b >= 0).

Sequence in context: A177925 A161462 A259649 * A153842 A125136 A021989

Adjacent sequences: A217865 A217866 A217867 * A217869 A217870 A217871

Keyword

nonn

Author

V. Raman, Oct 13 2012

Status

approved