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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 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

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