A217335 - OEIS

%I #19 Aug 21 2024 16:37:57

%S 2681,3523,6953,8227,16817,26101,28533,28563,28617,29011,34249,37007,

%T 42401,49983,50117,55703,60657,65083,66938,71381,71873,73443,76247,

%U 92773,92978,101109,101271,109129,111479,112051,113018,113721,115586,116267,119969,124177

%N Semiprimes p such that next semiprime after p is p+20.

%H Zak Seidov, <a href="/A217335/b217335.txt">Table of n, a(n) for n = 1..2000</a>

%e 2681 = A001358(760)  = 7*383, 2701 = A001358(761) = 37*73,

%e 3523 = A001358(986)  = 13*271, 3543 = A001358(987) = 3*1181.

%t f = Flatten@Position[Differences@(s = Select[Range@100000, PrimeOmega@# == 2 &]), 20]; s[[f]] (* _Hans Rudolf Widmer_, Aug 19 2024 *)

%o (Magma) IsSemiprime:=func<n | &+[d[2]: d in Factorization(n)] eq 2>; [n: n in [4..130000] | IsSemiprime(n) and IsSemiprime(n+20) and forall{n+i: i in [1..19] | not IsSemiprime(n+i)}]; // _Bruno Berselli_, Oct 01 2012

%Y Cf. A001358, A065516, A217030.

%K nonn

%O 1,1

%A _Zak Seidov_, Oct 01 2012