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 A158901 #16 Feb 08 2022 23:15:47
%S A158901 1,2,3,5,5,9,7,12,11,15,11,23,13,21,21,27,17,34,19,37,29,33,23,53,29,
%T A158901 39,37,51,29,65,31,58,45,51,45,83,37,57,53,83,41,89,43,79,73,69,47,
%U A158901 115,55,88,69,93,53,113,69,113,77,87,59,157,61,93,99,121,81,137,67,121,93,137
%N A158901 A051731 * (1, 1, 2, 3, 4, 5, ...).
%C A158901 a(n) = prime(n) if n is prime but nonprime n's can also have prime a(n).
%C A158901 Equals left border of triangle A158902.
%F A158901 A051731 * [1, 1, 2, 3, 4, 5, ...] = inverse Mobius transform of [1, 1, 2, 3, 4, ...].
%F A158901 a(n) = sigma(n) - d(n) + 1. - _Juri-Stepan Gerasimov_, Aug 30 2009
%F A158901 a(n) = 1 + A065608(n). - _R. J. Mathar_, Jan 08 2015
%e A158901 a(4) = 5 = (1, 1, 0, 1) dot (1, 1, 2, 3) = (1 + 1 + 0 + 3); where (1, 1, 0, 1) = row 4 of triangle A051731.
%p A158901 L := [1,seq(n, n=1..100)] ; read("transforms"); MOBIUSi(L) ; # _R. J. Mathar_, Apr 02 2009
%o A158901 (PARI) a(n) = sigma(n) - numdiv(n) + 1; \\ _Michel Marcus_, Sep 14 2017
%Y A158901 Cf. A158902.
%K A158901 nonn,easy
%O A158901 1,2
%A A158901 _Gary W. Adamson_, Mar 29 2009