Year-end appeal: Please make a donation to the OEIS Foundation to support ongoing development and maintenance of the OEIS. We are now in our 61st year, we have over 378,000 sequences, and we’ve reached 11,000 citations (which often say “discovered thanks to the OEIS”).
%I #15 Jan 27 2022 21:06:40
%S 18,27,30,36,42,45,50,54,60,63,66,70,72,75,78,81,84,90,98,99,100,102,
%T 105,108,110,114,117,120,125,126,130,132,135,138,140,144,147,150,153,
%U 154,156,162,165,168,170,171,174,175,180,182,186,189,190,195,196,198,200,204,207
%N Numbers k with at least one odd semiprime divisor < k.
%C Also, numbers with at least 1 partition into two parts (s,t), s<=t, such that s is an odd semiprime divisor of t.
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/ProperDivisor.html">Proper divisors</a>
%H Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/Semiprime.html">Semiprime</a>
%e 18 is in the sequence since 9 | 18, and 9 is an odd semiprime < n.
%e 30 is in the sequence since 15 | 30, and 15 is an odd semiprime < n.
%t q[n_] := If[EvenQ[n], PrimeOmega[n/2^IntegerExponent[n, 2]] > 1, PrimeOmega[n] > 2]; Select[Range[200], q] (* _Amiram Eldar_, Jan 23 2022 *)
%o (PARI) isok(m) = sumdiv(m, d, if ((d<m) && (d%2), bigomega(d)==2)) > 0; \\ _Michel Marcus_, Jan 23 2022
%Y Cf. A001358 (semiprimes), A032741, A086971.
%K nonn
%O 1,1
%A _Wesley Ivan Hurt_, Jan 23 2022