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 A061352 #2 Dec 05 2013 19:54:50 %S A061352 1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2, %T A061352 0,1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9, %U A061352 5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5,6,8,9,5,6,6,7,1,2,0,1,3,4,0,1,1,2,6,7,5 %N A061352 First row of array shown below. %C A061352 Least significant digit of the n-th triangular number if n is odd. %F A061352 a(1)=1; if n odd, a(n+1)=(a(n)+1) mod 10; if n even, a(n+1)=(a(n)+2n) mod 10. %F A061352 a(2n-1) = n*(2n-1)- 10*Floor[n*(2n-1)/10], a(2n) = n(2n+1)- 10*Floor[n*(2n+1)/10] + 1. %e A061352 1 2 6 7 5 6 8 9 5 ... %e A061352 3 5 8 4 7 7 0 4 ... %e A061352 4 9 3 8 6 1 3 ... %e A061352 0 2 9 5 2 2 ... %e A061352 1 0 4 3 1 ... %e A061352 1 3 4 0 ... %Y A061352 Cf. A061353. %K A061352 nonn,easy %O A061352 1,2 %A A061352 _Amarnath Murthy_, Apr 28 2001 %E A061352 More terms and formula from Larry Reeves (larryr(AT)acm.org), May 15 2001