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A216511 Number of positive integer solutions to the equation a^2 + 7*b^2 = n. 2
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 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,32

REFERENCES

Mohammad K. Azarian, Diophantine Pair, Problem B-881, Fibonacci Quarterly, Vol. 37, No. 3, August 1999, pp. 277-278.  Solution published in Vol. 38, No. 2, May 2000, pp. 183-184.

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-Fran├žois 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

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Last modified June 17 05:19 EDT 2019. Contains 324183 sequences. (Running on oeis4.)