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 A061581 #12 Oct 27 2023 22:05:37
%S A061581 1,2,4,8,16,212,424,848,16816,21216212,424212424,848424848,
%T A061581 1681684816816,212162121681621216212,42421242421216212424212424,
%U A061581 848424848424212424848424848,16816848168168484248481681684816816
%N A061581 a(1) = 1, a(n) = number obtained by replacing each digit of a(n-1) with its double.
%C A061581 It might have been more natural to start with index / offset 0. - _M. F. Hasler_, Jun 24 2016
%H A061581 Alois P. Heinz, <a href="/A061581/b061581.txt">Table of n, a(n) for n = 1..29</a>
%e A061581 The term after 16 is 2.12, i.e., 212.
%p A061581 a:= proc(n) option remember; `if`(n=1, 1, (l->
%p A061581 parse(cat(seq(2*l[-i], i=1..nops(l)))))(
%p A061581 convert(a(n-1), base, 10)))
%p A061581 end:
%p A061581 seq(a(n), n=1..20); # _Alois P. Heinz_, Jun 24 2016
%t A061581 NestList[FromDigits[Flatten[IntegerDigits[2*IntegerDigits[#]]]] &,1,16] (* _Jayanta Basu_, May 20 2013 *)
%o A061581 (PARI) A061581(n=2,a=1,m=2)={while(n--,a=eval(concat(apply(t->Str(t),digits(a)*m))));a} \\ If only the 2nd argument is given, then the operation is applied once to that argument. - _M. F. Hasler_, Jun 24 2016
%Y A061581 Cf. A061582 - A061587.
%K A061581 nonn,base,easy
%O A061581 1,2
%A A061581 _Amarnath Murthy_, May 13 2001
%E A061581 More terms from Larry Reeves (larryr(AT)acm.org) and _Asher Auel_, May 15 2001