A216511 - OEIS

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

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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