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

1,1

Links

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

Formula

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

Example

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

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

Crossrefs

Cf. A001358, A004086, A085778, A115670.

Sequence in context: A284439 A364664 A119656 * A339141 A180006 A259085

Adjacent sequences: A368235 A368236 A368237 * A368239 A368240 A368241

Keyword

nonn,base

Author

Zak Seidov and Robert Israel, Dec 18 2023

Status

approved