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 A262812 #18 Oct 11 2015 11:27:27 %S A262812 4,6,9,10,16,25,45,48,50,64,86,122,192,256,289,314,326,448,562,578, %T A262812 706,722,729,794,842,1226,1346,1458,1514,1681,1754,1922,2186,2401, %U A262812 2566,2601,2916,3362,3481,3866,3986,4046,4096,4274,4852,5043,5186,5414 %N A262812 Values of n such that sigma(n!) mod sigma(n) is not 0. %C A262812 Subsequence of A002808. %C A262812 Motivation was the investigation of sum of divisors of n! in terms of sum of divisors of n. %C A262812 Obviously, a(n) cannot be a prime number, although it can be a semiprime number. %C A262812 Is this sequence infinite? %F A262812 A066247(a(n)) = 1. %F A262812 A000005(a(n)) > 2. %F A262812 A001222(a(n)) > 1. %e A262812 a(1) = 4 because sigma(4!) mod sigma(4) = 60 mod 7 = 4. %e A262812 a(2) = 6 because sigma(6!) mod sigma(6) = 2418 mod 12 = 6. %e A262812 a(3) = 9 because sigma(9!) mod sigma(9) = 1481040 mod 13 = 2. %o A262812 (PARI) for(n=1, 1e30, if( sigma(n!) % sigma(n) != 0 , print1(n", "))) %Y A262812 Cf. A062569, A000203. %K A262812 nonn %O A262812 1,1 %A A262812 _Altug Alkan_, Oct 03 2015