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 A241535 #20 Mar 28 2015 16:02:36
%S A241535 0,0,4,4,0,4,9,4,21,9,4,9,25,4,9,15,25,4,15,9,4,15,21,9,9,15,4,9,4,9,
%T A241535 33,9,9,4,9,4,9,21,15,25,35,4,21,4,33,4,9,9,9,4,15,9,4,9,9,9,9,4,9,9,
%U A241535 4,21,25,25,4,51,33,25,9,4,9,15,21,9,9,21,15,9,9,15,21,4,21,4
%N A241535 Smallest semiprime q such that 2*prime(n) - q is semiprime, or a(n)=0 if there is no such q.
%C A241535 Conjecture: every even semiprime more than 22 is a sum of two semiprimes.
%C A241535 All terms are either 4 or odd.
%C A241535 First occurrence of k-th odd semiprime (A046315): 7, 16, 9, 13, 31, 41, 104, 134, 66, 412, 2769, 447, 1282, 3868, 7003, 3601, 48649, 11016, 5379, 41644, 34575, 83474, 120165, 135566, 21335, 394140, 14899, 876518, 434986, 173914, 691409, 1854580, 3741206, 714807, 1001321, 6427837, 4267513, 14809496, 7795998, 26617567, 2001937, 13958857, 9217135, 18815676, ..., . - _Robert G. Wilson v_, Apr 26 2014
%C A241535 If a(n) = 4, then 2*prime(n)-4=2*(prime(n)-2) is a semiprime, thus prime(n)-2 is a prime, so prime(n) belongs to A006512 (greater of twin primes). - _Michel Marcus_, Mar 26 2015
%H A241535 Robert G. Wilson v, <a href="/A241535/b241535.txt">Table of n, a(n) for n = 1..1000</a>
%e A241535 Let n=16, then 2*prime(16) = 2*53 = 106. We have 106-4=102, 106-6=100, 106-9=97, 106-10=96, 106-14=92, 106-15=91 and only the last number is semiprime. So a(16)=15.
%t A241535 NextSemiPrime[n_, k_: 1] := Block[{c = 0, sgn = Sign[k]}, sp = n + sgn; While[c < Abs[k], While[ PrimeOmega[sp] != 2, If[ sgn < 0, sp--, sp++]]; If[ sgn < 0, sp--, sp++]; c++]; sp + If[sgn < 0, 1, -1]]; f[n_] := Block[{p2 = 2 Prime[n], sp = 4}, While[ PrimeOmega[p2 - sp] != 2, sp = NextSemiPrime[sp]]; If[ sp != p2, sp, 0]]; Array[f, 75] (* _Robert G. Wilson v_, Apr 25 2014 *)
%Y A241535 Cf. A001358, A241531, A241533.
%K A241535 nonn
%O A241535 1,3
%A A241535 _Vladimir Shevelev_, Apr 25 2014
%E A241535 More terms from _Robert G. Wilson v_, Apr 25 2014