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 A193071 #16 Aug 18 2019 04:32:31 %S A193071 1,7,9,11,13,15,19,21,23,25,29,31,33,35,37,39,43,45,47,51,53,55,57,61, %T A193071 63,65,67,69,73,75,77,79,81,83,85,87,91,93,95,97,99,103,105,107,109, %U A193071 111,113,115,117,119,121,123,127,129,133,135,137,139,141,143,145,147 %N A193071 Odd numbers N for which sigma(N^2) is not prime. %C A193071 The function sigma(n) (=A000203(n)) takes odd values when n is a square or twice a square. Thus, odd numbers n for which sigma(n) is prime (cf. A023194) must be odd squares. This sequence gives the odd numbers whose square yields a composite sum of divisors (or 1). %C A193071 This is the complement of A193070 in the odd numbers A005408. %H A193071 Amiram Eldar, <a href="/A193071/b193071.txt">Table of n, a(n) for n = 1..10000</a> %F A193071 A193071 = A005408 \ A193070. %t A193071 Select[Range[1,151,2],!PrimeQ[DivisorSigma[1,#^2]]&] (* _Harvey P. Dale_, Dec 13 2012 *) %o A193071 (PARI) forstep(N=1, 1e7, 2, !isprime(sigma(N^2)) && print1(N", ")) %K A193071 nonn %O A193071 1,2 %A A193071 _M. F. Hasler_, Jul 15 2011