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 A084117 #6 Mar 31 2012 21:04:35 %S A084117 0,0,1,0,1,0,3,2,3,2,5,0,3,2,5,4,5,4,7,2,5,0,11,6,9,8,11,6,9,8,11,10, %T A084117 11,10,13,8,11,6,17,12,15,10,21,0,11,6,17,12,15,14,17,12,15,10,21,16, %U A084117 19,18,21,16,19,18,21,20,21,20,23,18,21,16,27,22,25,20,31,10,21,16,27,22,25 %N A084117 Multiplicity of the factor (1+x+x^2) in the characteristic polynomial modulo 2 of the symmetric matrix with entries binomial(i+j,i), 0<=i,j<n. %D A084117 R. Bacher and R. Chapman, Symmetric Pascal matrices modulo p, European J. Combin. 25 (2004), no. 4, 459-473. %F A084117 a(2n) = n - a(n). - _Ralf Stephan_, Oct 07 2003 %Y A084117 Cf. A085015. %K A084117 easy,nonn %O A084117 0,7 %A A084117 _Roland Bacher_, Jun 18 2003