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 A180116 #9 Apr 18 2019 02:54:25 %S A180116 1,3,33,55,555,777,7777,9999,99999,1111111111,111111111111, %T A180116 131313131313,13131313131313,15151515151515,1515151515151515, %U A180116 1717171717171717,171717171717171717,191919191919191919,19191919191919191919,21212121212121212121,2121212121212121212121 %N A180116 A008619(n-1)-fold concatenation of A109613(n). %C A180116 Written underneath, the first terms have increasing lengths filling a triangular shape: %C A180116 1, %C A180116 3, %C A180116 33, %C A180116 55, %C A180116 555, %C A180116 777, %C A180116 ... %p A180116 cat2 := proc(a,b) a*10^(max(1,1+ilog10(b)))+b ; end proc: %p A180116 A008619 := proc(n) 1+floor(n/2) ; end proc: %p A180116 A109613 := proc(n) 2*floor(n/2)+1 ; end proc: %p A180116 A180116 := proc(n) a := A109613(n) ; for t from 2 to A008619(n-1) do a := cat2(a,A109613(n)) ; end do: a ; end proc: %p A180116 seq(A180116(n),n=1..24) ; # _R. J. Mathar_, Sep 19 2010 %Y A180116 Cf. A000217, A109613, A008619. %K A180116 nonn,base,less %O A180116 1,2 %A A180116 _Mark Dols_, Aug 10 2010 %E A180116 Edited by _R. J. Mathar_, Sep 19 2010