cp's OEIS Frontend

A228920 Number of solutions to Sum_{i=1..n} x_i^2 == 0 (mod 4) with x_i in 0..3.

%I A228920 #28 Dec 21 2019 18:18:31
%S A228920 2,4,8,32,192,1024,4608,18432,69632,262144,1015808,4063232,16515072,
%T A228920 67108864,270532608,1082130432,4311744512,17179869184,68585259008,
%U A228920 274341036032,1098437885952,4398046511104,17600775979008,70403103916032,281543696187392
%N A228920 Number of solutions to Sum_{i=1..n} x_i^2 == 0 (mod 4) with x_i in 0..3.
%H A228920 Colin Barker, <a href="/A228920/b228920.txt">Table of n, a(n) for n = 1..1000</a>
%H A228920 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (8,-24,32).
%F A228920 a(n) = ((2+2i)^n + (2-2i)^n + 4^n)/4. - _Charles R Greathouse IV_, Sep 15 2013
%F A228920 G.f.: -2*x*(12*x^2-6*x+1) / ((4*x-1)*(8*x^2-4*x+1)). - _Colin Barker_, Nov 10 2014
%t A228920 Table[((2 + 2I)^n + (2 - 2I)^n + 4^n)/4, {n, 1, 30}]
%o A228920 (PARI) a(n)=((2+2*I)^n+(2-2*I)^n+4^n)/4 \\ _Charles R Greathouse IV_, Sep 15 2013
%o A228920 (PARI) Vec(-2*x*(12*x^2-6*x+1)/((4*x-1)*(8*x^2-4*x+1)) + O(x^100)) \\ _Colin Barker_, Nov 10 2014
%Y A228920 Cf. A101990, A228921, A229136, A318609, A318610.
%Y A228920 Column k = 0 of A330619.
%K A228920 nonn,easy
%O A228920 1,1
%A A228920 _José María Grau Ribas_, Sep 15 2013
%E A228920 a(13)-a(25) from _Charles R Greathouse IV_, Sep 15 2013