A191912 Semiprimes p*q such that the concatenation p||q or q||p is again a semiprime.

4, 9, 10, 15, 25, 26, 34, 35, 38, 39, 49, 55, 57, 69, 74, 85, 87, 91, 94, 95, 106, 115, 118, 119, 121, 123, 134, 141, 145, 159, 161, 169, 178, 183, 185, 187, 202, 205, 206, 209, 213, 215, 217, 221, 237, 254, 259, 265, 267, 287, 289, 291, 295, 298, 301, 303, 309, 314, 319, 321, 334, 339, 346, 361, 362, 365, 371, 377, 381, 382, 391

Offset

1,1

Links

Table of n, a(n) for n=1..71.

Example

The semiprime 39=3*13 is not in A191911 since the concatenation 3||13 = 313 is not a semiprime, but reversing the order of the factors yields the semiprime 13||3 = 133 = 7*19, which makes 39 a member of this sequence.

Prog

(PARI) for(i=1, 500, bigomega(i)==2|next; f=factor(i)~; (bigomega(eval(Str(f[1, 1], f[1, #f])))==2 | bigomega(eval(Str(f[1, 1], f[1, #f])))==2) & print1(i", "))

Crossrefs

Contains A191911 as a subsequence.

Sequence in context: A110602 A254923 A122635 * A191911 A329476 A164098

Adjacent sequences: A191909 A191910 A191911 * A191913 A191914 A191915

Keyword

nonn

Author

M. F. Hasler, Jun 19 2011

Status

approved