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