cp's OEIS Frontend

A216282 Number of nonnegative solutions to the equation x^2 + 2*y^2 = n.

%I A216282 #20 Aug 23 2017 09:49:56
%S A216282 1,1,1,1,0,1,0,1,2,0,1,1,0,0,0,1,1,2,1,0,0,1,0,1,1,0,2,0,0,0,0,1,2,1,
%T A216282 0,2,0,1,0,0,1,0,1,1,0,0,0,1,1,1,2,0,0,2,0,0,2,0,1,0,0,0,0,1,0,2,1,1,
%U A216282 0,0,0,2,1,0,1,1,0,0,0,0,3,1,1,0,0,1,0,1,1,0,0,0,0,0,0,1,1,1,3,1,0,2,0,0,0,0,1,2,0,0,0,0,1,2,0,0,0,1,0,0
%N A216282 Number of nonnegative solutions to the equation x^2 + 2*y^2 = n.
%C A216282 Records occur at 1, 9, 81, 297, 891, 1683, 5049, 15147, 31977, ... - _Antti Karttunen_, Aug 23 2017
%H A216282 Antti Karttunen, <a href="/A216282/b216282.txt">Table of n, a(n) for n = 1..65537</a>
%e A216282 For n = 9, there are two solutions: 9 = 9^2 + 2*(0^2) = 1^2 + 2*(2^2), thus a(9) = 2.
%e A216282 For n = 81, there are three solutions: 81 = 9^2 + 2*(0^2) = 3^2 + 2*(6^2) = 7^2 + 2*(4^2), thus a(81) = 3.
%e A216282 For n = 65536, there is one solution: 65536 = 256^2 + 2*(0^2) = 65536 + 0, thus a(65536) = 1.
%e A216282 For n = 65537, there is one solution: 65537 = 255^2 + 2*(16^2) = 65205 + 512, thus a(65537) = 1.
%t A216282 r[n_] := Reduce[x >= 0 && y >= 0 && x^2 + 2 y^2 == n, Integers];
%t A216282 a[n_] := Which[rn = r[n]; rn === False, 0, Head[rn] === And, 1, Head[rn] === Or, Length[rn], True, -1];
%t A216282 Table[a[n], {n, 1, 120}] (* _Jean-François Alcover_, Jun 24 2017 *)
%o A216282 (Scheme) (define (A216282 n) (cond ((< n 2) 1) (else (let loop ((k (- (A000196 n) (modulo (- n (A000196 n)) 2))) (s 0)) (if (< k 0) s (let ((x (/ (- n (* k k)) 2))) (loop (- k 2) (+ s (A010052 x))))))))) ;; _Antti Karttunen_, Aug 23 2017
%Y A216282 Cf. A092573, A119395, A000161, A025426, A216283.
%Y A216282 Cf. A002479 (positions of nonzeros), A097700 (of zeros).
%K A216282 nonn
%O A216282 1,9
%A A216282 _V. Raman_, Sep 03 2012
%E A216282 Examples from _Antti Karttunen_, Aug 23 2017