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 A267894 #21 Dec 03 2020 06:58:19 %S A267894 1,2,4,8,15,16,21,27,30,32,33,35,39,42,45,51,54,55,57,60,63,64,65,66, %T A267894 69,70,75,77,78,84,85,87,90,91,93,95,99,102,105,108,110,111,114,115, %U A267894 117,119,120,123,125,126,128,129,130,132,133,135,138,140,141,143,145,147,150 %N A267894 Numbers whose number of odd divisors is nonprime. %H A267894 Amiram Eldar, <a href="/A267894/b267894.txt">Table of n, a(n) for n = 1..10000</a> %e A267894 The divisors of 42 are 1, 2, 3, 6, 7, 14, 21, 42. The odd divisors of 42 are 1, 3, 7, 21. There are 4 odd divisors of 42 and 4 is a nonprime number, so 42 is in the sequence. %t A267894 Select[Range[150], !PrimeQ[DivisorSigma[0, #/2^IntegerExponent[#, 2]]] &] (* _Amiram Eldar_, Dec 03 2020 *) %o A267894 (PARI) isok(n) = ! isprime(sumdiv(n, d, (d%2))); \\ _Michel Marcus_, Apr 04 2016 %Y A267894 Cf. A000079, A001227, A018252, A028982, A028983, A131651, A230577, A266531, A267891, A267892, A267893, A267895. %K A267894 nonn %O A267894 1,2 %A A267894 _Omar E. Pol_, Apr 04 2016