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 A061353 #2 Dec 05 2013 20:10:44 %S A061353 1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9,5, %T A061353 6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6, %U A061353 7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1 %N A061353 First column of array shown in A061352. %C A061353 Least significant digit of the n-th triangular number if n is even. %F A061353 a(1)=1; if n even, a(n+1)=(a(n)+1) mod 10; if n odd, a(n+1)=(a(n)+2n) mod 10. %F A061353 a(2n-1) = n*(2n-1)- 10*Floor[n*(2n-1)/10] +1, a(2n) = n(2n+1)- 10*Floor[n*(2n+1)/10]. %K A061353 nonn,easy %O A061353 1,2 %A A061353 _Amarnath Murthy_, Apr 28 2001 %E A061353 More terms and formula from Larry Reeves (larryr(AT)acm.org), May 15 2001