cp's OEIS Frontend

A373731 Semiprimes k such that the digit reversal of k is a semiprime > k.

%I A373731 #10 Jun 23 2024 22:04:19
%S A373731 15,26,39,49,58,115,122,123,129,143,155,158,159,169,177,178,183,185,
%T A373731 187,203,205,226,265,289,314,319,326,327,329,335,339,355,394,398,415,
%U A373731 437,497,538,559,586,589,629,667,718,899,1006,1011,1027,1041,1043,1046,1047,1057,1059,1067,1079,1115,1119
%N A373731 Semiprimes k such that the digit reversal of k is a semiprime > k.
%H A373731 Robert Israel, <a href="/A373731/b373731.txt">Table of n, a(n) for n = 1..10000</a>
%e A373731 a(3) = 39 is a term because 39 = 3*13 is a semiprime, its reversal 93 = 3*31 is a semiprime, and 93 > 39.
%p A373731 rev:= proc(n) local L,i;
%p A373731   L:= convert(n,base,10);
%p A373731   add(L[-i]*10^(i-1),i=1..nops(L))
%p A373731 end proc:
%p A373731 filter:= proc(n) local r;
%p A373731   r:= rev(n);
%p A373731   r > n and numtheory:-bigomega(n) = 2 and numtheory:-bigomega(r) = 2
%p A373731 end proc:
%p A373731 select(filter, [$1..2000]);
%t A373731 s = {}; Do[fd = FromDigits[Reverse[IntegerDigits[k]]]; If[{2, 2} ==PrimeOmega[{fd, k}] && fd > k, AppendTo[s, k]], {k, 1000}]; s
%Y A373731 Cf. A001358, A004086, A050700, A097393.
%K A373731 nonn,base
%O A373731 1,1
%A A373731 _Zak Seidov_ and _Robert Israel_, Jun 17 2024