A085777 Semiprimes such that when they are concatenated with their reversal are semiprime.

14, 34, 38, 74, 91, 94, 115, 146, 169, 185, 194, 305, 334, 341, 346, 355, 361, 362, 386, 394, 395, 398, 706, 721, 731, 745, 755, 766, 794, 799, 901, 923, 926, 955, 974, 985, 1027, 1046, 1094, 1138, 1141, 1147, 1174, 1195, 1202, 1238, 1247, 1261, 1262, 1271

Offset

1,1

Comments

This is to A001358 = semiprimes (or biprimes): products of two primes, as A054217 is to primes A000040. Note that a subsequence is the emirpimes (A097393) which, when concatenated with their digital reversals, become different semiprimes. Examples of this subsequence: 94, 115, 169, 185, ... because 9449 = 11 * 859, 115511 = 11 * 10501, 169961 = 11 * 15451, 185581 = 11 * 16871. [From Jonathan Vos Post, Apr 05 2010]

Links

Table of n, a(n) for n=1..50.

Formula

{positive integers s = p*q in A001358, p and q primes, such that Concatenate(s, R(s)) = u*v is in A001358, u and v primes} = {positive integers s = p*q in A001358, p and q primes, such that Concatenate(s, A004086(s)) = u*v is in A001358, u and v primes}. [From Jonathan Vos Post, Apr 05 2010]

Example

a(7)=115 because 115511 = 11*10501.

Crossrefs

Cf. A001358.

Cf. A000040, A001358, A004086, A097393 [From Jonathan Vos Post, Apr 05 2010]

Sequence in context: A112878 A175559 A158899 * A329062 A072566 A039449

Adjacent sequences: A085774 A085775 A085776 * A085778 A085779 A085780

Keyword

base,nonn

Author

Jason Earls, Jul 23 2003

Status

approved