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 A061520 #11 Jan 10 2021 14:17:41
%S A061520 0,6,12,78,1314,79710,13151376,79711791312,1315137713157978,
%T A061520 797117913137971113151314,1315137713157979131513777971179710,
%U A061520 797117913137971113151315797117913131315137713151376
%N A061520 a(0) = 0; a(n) is obtained by incrementing each digit of a(n-1) by 6.
%C A061520 In A061511-A061522, A061746-A061750 when the incremented digit exceeds 9 it is written as a 2-digit string. So 9+1 becomes the 2-digit string 10, etc.
%H A061520 Robert Israel, <a href="/A061520/b061520.txt">Table of n, a(n) for n = 0..17</a>
%p A061520 g:= proc(n) op(convert(n+6,base,10)) end proc:
%p A061520 L[0]:= [0]:
%p A061520 for n from 1 to 12 do L[n]:= map(g,L[n-1]) od:
%p A061520 map(t -> add(t[i]*10^(i-1),i=1..nops(t)), [seq(L[i],i=0..12)]): # _Robert Israel_, Jan 26 2020
%t A061520 Nest[Append[#, FromDigits@ Flatten@ IntegerDigits[IntegerDigits@ #[[-1]] + 6]] &, {0}, 11] (* _Michael De Vlieger_, Jan 26 2020 *)
%t A061520 NestList[FromDigits[Flatten[IntegerDigits/@(IntegerDigits[#]+6)]]&,0,15] (* _Harvey P. Dale_, Jan 10 2021 *)
%K A061520 base,nonn
%O A061520 0,2
%A A061520 _Amarnath Murthy_, May 08 2001
%E A061520 More terms from Larry Reeves (larryr(AT)acm.org), May 11 2001
%E A061520 Corrected by _Robert Israel_, Jan 26 2020