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 A137287 #8 Mar 15 2022 07:01:16 %S A137287 2,21,221,2221,222221,2222221,222222221,2222222221,222222222221, %T A137287 222222222222221,2222222222222221,2222222222222222221, %U A137287 222222222222222222221,2222222222222222222221,222222222222222222222221,222222222222222222222222221,222222222222222222222222222221 %N A137287 a(n) is the number 2 written (prime(n)-1)/2 times followed by the digit 1; a(1)=2. %C A137287 Sum of digits of a(n) = prime(n). %F A137287 A007953(a(n)) = A000040(n). - _Michel Marcus_, Mar 15 2022 %e A137287 prime(7)=17; (17-1)/2=8; a(7) = 222222221, 8*"2" concatenated with "1". %o A137287 (PARI) a(n) = if (n==1, 2, fromdigits(concat(vector((prime(n)-1)/2, k, 2), 1))); \\ _Michel Marcus_, Mar 15 2022 %Y A137287 Cf. A000040, A031974. %K A137287 easy,nonn,base %O A137287 1,1 %A A137287 _Ctibor O. Zizka_, Apr 05 2008 %E A137287 More terms from _Michel Marcus_, Mar 15 2022