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 A001731 #16 Feb 02 2022 08:20:34 %S A001731 9,10,11,12,13,14,21,100,1001,111111111 %N A001731 a(n) is 9 written in base 10-n. %t A001731 Append[Array[FromDigits@ IntegerDigits[9, 10 - #] &, 9, 0], FromDigits@ ConstantArray[1, 9]] (* _Michael De Vlieger_, Feb 02 2022 *) %o A001731 (PARI) a(n) = if (n==9, 111111111, fromdigits(digits(9, 10-n), 10)); \\ _Michel Marcus_, Feb 02 2022 %Y A001731 Cf. A001732, A001733, A001734, A001735, A001736. %K A001731 fini,nonn,base,full %O A001731 0,1 %A A001731 _N. J. A. Sloane_ %E A001731 Corrected by _Henry Bottomley_, Nov 23 2000