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 A280405 #11 Jan 09 2017 20:57:06
%S A280405 9,33,51,69,87,111,123,141,159,177,201,213,237,249,267,291,303,321,
%T A280405 339,381,393,411,447,471,489,501,519,537,573,591,633,669,681,699,717,
%U A280405 753,771,789,807,831,843,879,921,933,951,993
%N A280405 Odd semiprimes that cannot be represented as 2p+3q, where p and q are primes.
%H A280405 Robert Israel, <a href="/A280405/b280405.txt">Table of n, a(n) for n = 1..10000</a>
%e A280405 33 = 3*11 is a semiprime, and cannot be represented as twice a prime plus three times a prime. 21=3*7 is a semiprime which CAN be represented in that form, i.e. 2*3+3*5, and thus is not in this sequence.
%p A280405 N:= 10^4: # to get all terms <= N
%p A280405 Primes:= select(isprime,[2,seq(i,i=3..N/2,2)]):
%p A280405 Cands:= {seq(i,i=1..N,2)} minus {seq(seq(2*p+3*q,p=Primes),q=Primes)}:
%p A280405 sort(convert(select(numtheory:-bigomega=2, Cands),list)); # _Robert Israel_, Jan 09 2017
%Y A280405 Cf. A046315 (odd semiprimes)
%Y A280405 Cf. A280389 (odd semiprimes which can be represented as 2p+3q, where p and q are prime)
%K A280405 nonn
%O A280405 1,1
%A A280405 _Randy L. Ekl_, Jan 02 2017