OFFSET
1,1
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
KEYWORD
nonn
AUTHOR
M. F. Hasler, Jun 19 2011
STATUS
approved