A216511 Number of positive integer solutions to the equation a^2 + 7*b^2 = n.

0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 0, 0, 2, 0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 1, 1, 0, 0, 0, 0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 1, 0, 0, 0, 1, 1, 0, 0, 0, 0, 1, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0

Offset

1,32

Links

Table of n, a(n) for n=1..87.

Mathematica

r[n_] := Reduce[x > 0 && y > 0 && x^2 + 7 y^2 == n, Integers];
a[n_] := Which[rn = r[n]; rn === False, 0, Head[rn] === And, 1, Head[rn] === Or, Length[rn], True, -1];
Table[a[n], {n, 1, 87}] (* Jean-François Alcover, Jun 24 2017 *)

Crossrefs

Cf. A155717.

Sequence in context: A086077 A178408 A073345 * A138088 A112765 A105966

Adjacent sequences: A216508 A216509 A216510 * A216512 A216513 A216514

Keyword

nonn

Author

V. Raman, Sep 08 2012

Status

approved