A383468 Semiprimes s = A001358(k) such that k, s - k and s + k are also semiprimes.

10, 15, 141, 166, 274, 298, 299, 687, 995, 1115, 1227, 1299, 1345, 1891, 1945, 2194, 2661, 2998, 3093, 3287, 3566, 3781, 3902, 4174, 4262, 4497, 4798, 5378, 5414, 5758, 6609, 7094, 7666, 8354, 8434, 9566, 10041, 10342, 11051, 11091, 11486, 11582, 11702, 12279, 12574, 13154, 13346, 13387, 13466

Offset: 1

Author

Zak Seidov and Robert Israel, Apr 27 2025

Keywords

• nonn

Comments

Except for a(1) = 10 = A001358(4), s and k always have different parities.

Examples

			a(3) = 141 is a term because 141 = 3 * 47 = A001358(46) is a semiprime and 46 = 2 * 23, 141 - 46 = 95 = 5 * 19 and 141 + 46 = 187 = 11 * 17 are all semiprimes.
		

Links

• Robert Israel, Table of n, a(n) for n = 1..10000

Crossrefs

Cf. A001358. A383469.

Programs

Maple

k:= 0: R:= NULL: count:= 0:
for s from 1 while count < 100 do
  if numtheory:-bigomega(s) = 2 then
    k:= k+1;
    if andmap(t -> numtheory:-bigomega(t) = 2, [k, s-k, s+k]) then
      R:= R, s; count:= count+1;
    fi
  fi;
od:
R;

Formula

a(n) = A001358(A383469(n)).