A191911 Semiprimes p*q such that the concatenation p||q is again a semiprime, where p <= q.

4, 9, 10, 15, 25, 26, 34, 35, 38, 49, 55, 57, 69, 74, 85, 87, 91, 94, 95, 106, 118, 119, 121, 123, 134, 145, 161, 169, 178, 183, 185, 202, 206, 209, 213, 215, 217, 221, 254, 259, 265, 289, 295, 298, 303, 309, 314, 321, 334, 339, 346, 361, 362, 365, 371, 377, 381, 382, 393, 395, 398, 407, 415, 417, 437, 445, 447, 451, 453, 454, 458

Offset

1,1

Links

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

Example

a(1)=4 because 4=2*2 and 22=2*11 are semiprimes.
The semiprime 6=2*3 is not in the sequence, because the concatenation 2||3 = 23 is not a semiprime.
a(2)=9 because 9=3*3 and 33=3*11 are semiprimes.
a(3)=10 because 10=2*5 and 25=5*5 are semiprimes.
The semiprime 14=2*7 is not in the sequence, because 2||7 = 27 = 3*3*3 is not a semiprime.
a(4)=15 because 15=3*5 and 35=5*7 are semiprimes.

Prog

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

Crossrefs

Cf. A001358.

Sequence in context: A254923 A122635 A191912 * A329476 A164098 A007936

Adjacent sequences: A191908 A191909 A191910 * A191912 A191913 A191914

Keyword

nonn

Author

M. F. Hasler, Jun 19 2011

Status

approved