A368238 Semiprimes whose reversal is a prime, ordered by the prime.

91, 14, 34, 74, 35, 95, 38, 301, 901, 721, 731, 361, 371, 391, 791, 922, 142, 362, 382, 703, 713, 133, 943, 763, 973, 793, 914, 134, 334, 934, 974, 194, 305, 905, 145, 745, 755, 365, 965, 785, 395, 995, 106, 706, 146, 346, 746, 166, 386, 917, 377, 118, 358, 758, 958, 778, 119, 749, 779, 799, 3101

Offset: 1

Author

Zak Seidov and Robert Israel, Dec 18 2023

Keywords

• nonn
• base

Examples

			a(4) = 74 because A115670(4) = 47 is the 4th prime whose reversal is a semiprime, and 74 is that reversal.
		

Links

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

Crossrefs

Cf. A001358, A004086, A085778, A115670.

Programs

Maple

rev:= proc(n) local L,i;
       L:= convert(n,base,10);
       add(L[-i]*10^(i-1),i=1..nops(L))
end proc:
map(rev, select(p -> isprime(p) and numtheory:-bigomega(rev(p)) = 2, [seq(i,i=3..1000,2)]);

Mathematica

s = {}; Do[If[2 == PrimeOmega[sm = FromDigits[Reverse[IntegerDigits[Prime[k]]]]], AppendTo[s, sm]], {k, 200}]; s

Formula

a(n) = A004086(A085778(n)).