OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..300
EXAMPLE
77 is in the sequence because (1 )it is a semiprime and (2) the product of its digits 7*7=49 is also a semiprime.
MATHEMATICA
Select[Range[2000], PrimeOmega[#]==PrimeOmega[Times@@IntegerDigits[#]] == 2&] (* Harvey P. Dale, Dec 27 2013 *)
PROG
(PARI) A007954(n)= { local(resul) ; if(n==0, return(0) ; ) ; resul= 1 ; while(n>0, resul *= n%10 ; n = (n-n%10)/10 ; ) ; return(resul) ; } { for(n=4, 2000, if( bigomega(n)==2, if(A007954(n) != 0 && bigomega(A007954(n)) == 2, print1(n, ", "); ) ; ) ; ) ; } - R. J. Mathar, Aug 21 2006
CROSSREFS
KEYWORD
base,nonn
AUTHOR
Luc Stevens (lms022(AT)yahoo.com), May 20 2006
STATUS
approved