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 A052246 #14 Sep 05 2025 23:46:16
%S A052246 0,10,210,3210,43210,543210,6543210,76543210,876543210,9876543210,
%T A052246 109876543210,11109876543210,1211109876543210,131211109876543210,
%U A052246 14131211109876543210,1514131211109876543210,161514131211109876543210,17161514131211109876543210
%N A052246 Concatenation of integers from n down to 0.
%F A052246 a(0)=0, a(n) = n*10^len(a(n-1)) + a(n-1), where len(k) = number of digits in k and len(0)=1.
%F A052246 a(n) = A000422(n)*10.
%p A052246 a:= proc(n) a(n):= `if`(n=0, 0, parse(cat(n, a(n-1)))) end:
%p A052246 seq(a(n), n=0..22); # _Alois P. Heinz_, Jan 12 2021
%t A052246 nn=20;With[{c=Range[nn,0,-1]},Table[FromDigits[Flatten[ IntegerDigits/@ Take[ c,-n]]],{n,nn}]] (* _Harvey P. Dale_, Feb 01 2013 *)
%o A052246 (PARI) a(n)=if(n==0, 0, eval(Str(n, a(n-1)))); \\ _Joerg Arndt_, Sep 14 2014
%Y A052246 Cf. A000422.
%K A052246 base,easy,nonn,changed
%O A052246 0,2
%A A052246 _Henry Bottomley_, Feb 01 2000