cp's OEIS Frontend

A383469 Semiprimes k such that A001358(k) + k and A001358(k) - k are also semiprimes.

%I A383469 #6 Apr 29 2025 13:28:09
%S A383469 4,6,46,55,87,93,94,206,298,326,362,382,394,542,562,629,758,841,866,
%T A383469 926,993,1046,1079,1147,1167,1234,1317,1469,1477,1561,1774,1895,2047,
%U A383469 2227,2245,2515,2638,2705,2894,2902,3007,3031,3063,3202,3269,3409,3453,3466,3487,3809,3891,4058,4174,4207
%N A383469 Semiprimes k such that A001358(k) + k and A001358(k) - k are also semiprimes.
%H A383469 Robert Israel, <a href="/A383469/b383469.txt">Table of n, a(n) for n = 1..10000</a>
%F A383469 A001358(a(k)) = A383468(k).
%e A383469 a(3) = 46 is a term because 46 = 2 * 23 is a semiprime, A001358(46) = 141, and 141 - 46 = 95 = 5 * 19 and 141 + 46 = 187 = 11 * 17 are semiprimes.
%p A383469 k:= 0: K:= NULL: count:= 0:
%p A383469 for s from 1 while count < 100 do
%p A383469   if numtheory:-bigomega(s) = 2 then
%p A383469     k:= k+1;
%p A383469     if andmap(t -> numtheory:-bigomega(t) = 2, [k, s-k, s+k]) then
%p A383469       K:= K, k; count:= count+1;
%p A383469     fi
%p A383469   fi;
%p A383469 od:
%p A383469 K;
%Y A383469 Cf. A001358, A383468.
%K A383469 nonn
%O A383469 1,1
%A A383469 _Zak Seidov_ and _Robert Israel_, Apr 27 2025