This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A131731 #18 Dec 12 2023 09:00:36 %S A131731 2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4, %T A131731 -3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3, %U A131731 4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3,2,-3,4,-3 %N A131731 Period 4: repeat [2, -3, 4, -3]. %H A131731 <a href="/index/Rec#order_03">Index entries for linear recurrences with constant coefficients</a>, signature (-1,-1,-1). %F A131731 From _Luce ETIENNE_, Mar 25 2017: (Start) %F A131731 a(n)= (6*(-1)^n-(-1)^((2*n+1-(-1)^n)/4)-(-1)^((2*n-1+(-1)^n)/4))/2. %F A131731 a(n) = 3*cos(n*Pi)-cos(n*Pi/2). (End) %K A131731 sign,easy %O A131731 0,1 %A A131731 _Paul Curtz_, Sep 17 2007 %E A131731 Better name from _Joerg Arndt_, Apr 02 2017