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 A061580 #15 Oct 27 2023 22:05:37 %S A061580 1,4,16,424,16816,42432424,1681612816816,42432424483242432424, %T A061580 168161281681616321281681612816816, %U A061580 42432424483242432424424128483242432424483242432424 %N A061580 a(1) = 1, a(n)= number obtained by replacing each digit of a(n-1) with four times its value. %H A061580 Michael De Vlieger, <a href="/A061580/b061580.txt">Table of n, a(n) for n = 1..16</a> %t A061580 NestList[FromDigits@ Flatten@ Map[IntegerDigits, 4*IntegerDigits[#]] &, 1, 9] (* _Michael De Vlieger_, Feb 25 2023 *) %Y A061580 Cf. A061581, A061582, A061583, A061584, A061585, A061586, A061587. %K A061580 nonn,base,easy %O A061580 1,2 %A A061580 _Amarnath Murthy_, May 13 2001 %E A061580 More terms from Larry Reeves (larryr(AT)acm.org) and _Asher Auel_, May 15 2001 %E A061580 a(9) corrected by _N. J. A. Sloane_, Jan 23 2017 at the suggestion of Carolyn Liao. %E A061580 a(10) corrected by _Sean A. Irvine_, Feb 25 2023