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 A192690 #31 Apr 22 2025 05:47:26 %S A192690 1,12,16,18,20,24,28,40,44,45,48,50,52,54,56,60,63,64,68,72,75,76,80, %T A192690 81,84,88,90,92,96,98,99,104,108,112,116,117,124,126,132,135,136,140, %U A192690 147,148,150,152,153,156,160,162,164,171,172,175,176,180,184,188 %N A192690 Nonprime numbers with a nonprime number of nonprime divisors. %e A192690 For example, 12 is composite and it has 6 divisors: 1, 2, 3, 4, 6, 12. Of these, 4 are not prime: 1, 4, 6, 12. Since 4 is not prime either, 12 is in the sequence. %t A192690 NonPrimeDivisors[n_] := Length[Select[Divisors[n], ! PrimeQ[#] &]]; Select[Range[200], ! PrimeQ[#] && ! PrimeQ[NonPrimeDivisors[#]] &] (* _T. D. Noe_, Oct 20 2011 *) %o A192690 (SageMath) %o A192690 def npd(n: int) -> int: %o A192690 return len([d for d in divisors(n) if not is_prime(d)]) %o A192690 def isA192690(n: int) -> bool: %o A192690 return not (is_prime(n) or is_prime(npd(n))) %o A192690 A192690List = lambda b: [n for n in range(1, b) if isA192690(n)] %o A192690 print(A192690List(189)) # _Peter Luschny_, Apr 22 2025 %Y A192690 Cf. A139118, A141468, A167175. %Y A192690 Cf. A018252. - Omar E. Pol, Oct 20 2011 %K A192690 nonn %O A192690 1,2 %A A192690 _Juri-Stepan Gerasimov_, Oct 15 2011