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 A256387 #10 Apr 13 2020 19:49:01 %S A256387 5,7,11,13,17,19,21,23,25,29,31,33,35,37,41,43,45,47,49,51,53,55,59, %T A256387 61,63,65,67,71,73,75,77,79,81,83,85,87,89,91,93,95,97,101,103,107, %U A256387 109,111,113,115,117,119,121,123,125,127,129,131,133,137,139,141,143 %N A256387 Numbers n such that no prime can be the arithmetic mean of 2 semiprimes whose difference is 2*n. %C A256387 That is, there is no prime p, such that p+n and p-n are both semiprime. %C A256387 Complement of A256389. %C A256387 From _Robert Israel_, Apr 13 2020: (Start) %C A256387 Includes odd number n if and only if n+4 is not prime or 2*n+4 is not a semiprime. %C A256387 There any no even members up to 10^5. Conjecture: all members are odd. (End) %H A256387 Robert Israel, <a href="/A256387/b256387.txt">Table of n, a(n) for n = 1..10000</a> %e A256387 A256383 is the list of numbers n such that n-5 and n+5 are semiprimes, and it contains no prime, hence 5 is in the sequence. %p A256387 select(t -> not isprime(t+4) or numtheory:-bigomega(2*t+4) <> 2, [seq(i,i=1..1000,2)]); # _Robert Israel_, Apr 13 2020 %Y A256387 Cf. A256383. %Y A256387 Cf. A256388 (a single prime), A256389 (one or more primes). %K A256387 nonn %O A256387 1,1 %A A256387 _Michel Marcus_, Mar 27 2015