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

15, 26, 39, 49, 58, 115, 122, 123, 129, 143, 155, 158, 159, 169, 177, 178, 183, 185, 187, 203, 205, 226, 265, 289, 314, 319, 326, 327, 329, 335, 339, 355, 394, 398, 415, 437, 497, 538, 559, 586, 589, 629, 667, 718, 899, 1006, 1011, 1027, 1041, 1043, 1046, 1047, 1057, 1059, 1067, 1079, 1115, 1119

Offset: 1

Author

Zak Seidov and Robert Israel, Jun 17 2024

Keywords

• nonn
• base

Examples

			a(3) = 39 is a term because 39 = 3*13 is a semiprime, its reversal 93 = 3*31 is a semiprime, and 93 > 39.
		

Links

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

Crossrefs

Cf. A001358, A004086, A050700, A097393.

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:
filter:= proc(n) local r;
  r:= rev(n);
  r > n and numtheory:-bigomega(n) = 2 and numtheory:-bigomega(r) = 2
end proc:
select(filter, [$1..2000]);

Mathematica

s = {}; Do[fd = FromDigits[Reverse[IntegerDigits[k]]]; If[{2, 2} ==PrimeOmega[{fd, k}] && fd > k, AppendTo[s, k]], {k, 1000}]; s