A118689 Semiprimes for which the product of the digits is also a semiprime.

4, 6, 9, 14, 22, 25, 33, 35, 55, 57, 77, 91, 119, 122, 123, 133, 141, 155, 161, 177, 213, 215, 217, 221, 321, 371, 411, 515, 517, 551, 611, 713, 717, 721, 731, 771, 1114, 1119, 1133, 1135, 1137, 1141, 1157, 1177, 1191, 1271, 1313, 1315, 1317, 1351, 1371, 1411

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

Cf. A001358.

Sequence in context: A175709 A073679 A257559 * A364489 A118692 A020706

Adjacent sequences: A118686 A118687 A118688 * A118690 A118691 A118692

Keyword

base,nonn

Author

Luc Stevens (lms022(AT)yahoo.com), May 20 2006

Status

approved