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 A155040 #3 Mar 30 2012 18:59:22
%S A155040 1,1,1,1,-1,1,1,-1,-1,1,1,-1,1,-1,1,1,-1,1,1,-1,1,1,-1,1,-1,1,-1,1,1,
%T A155040 -1,1,-1,-1,1,-1,1,1,-1,1,-1,1,-1,1,-1,1,1,-1,1,-1,1,1,-1,1,-1,1,1,-1,
%U A155040 1,-1,1,-1,1,-1,1,-1,1,1,-1,1,-1,1,-1,-1,1,-1,1,-1,1,1,-1,1,-1,1,-1,1,-1,1
%N A155040 A symmetric (1,-1)-triangle.
%C A155040 Row sums are A007877(n+1). Diagonal sums are A155041.
%F A155040 Number triangle T(n,k)=sum{j=0..n, [j<=k]*[j<=n-k]*(-1)*(((-1)^(j+1)-0^(j+1))-((-1)^j-0^j))}.
%e A155040 Triangle begins
%e A155040 .1,
%e A155040 .1, 1,
%e A155040 .1, -1, 1,
%e A155040 .1, -1, -1, 1,
%e A155040 .1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, 1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, 1, 1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, 1, -1, 1, -1, 1, -1, 1,
%e A155040 .1, -1, 1, -1, 1, -1, -1, 1, -1, 1, -1, 1
%K A155040 easy,sign,tabl
%O A155040 0,1
%A A155040 _Paul Barry_, Jan 19 2009