# Greetings from The On-Line Encyclopedia of Integer Sequences! http://oeis.org/ Search: id:a118688 Showing 1-1 of 1 %I A118688 #22 Jul 09 2015 06:02:24 %S A118688 4,6,9,15,22,33,46,51,55,69,77,82,86,87,91,95,118,121,123,141,145,158, %T A118688 159,177,185,194,202,213,217,226,235,249,253,262,267,301,303,321,329, %U A118688 334,339,361,365,393,411,415,437,446,447,451,473,482,489,501,505,514 %N A118688 Semiprimes for which the sum of the digits is also a semiprime. %C A118688 The first term congruent to 2 mod 9 is a(2729) = 29999. - _Robert Israel_, Jul 07 2015 %C A118688 Among first 10000 terms, numbers of terms congruent to {0..8} mod 9 are: {1,425,139,1453,2773,1233,1252,3087,2739}. Terms with minimal digitsum = 4 are: {4,22,121,202,301,1003,1111,2101,10003, 10021,10102,10201,11002,11101,12001,30001,100021,100102,100201,101011, 110002,110101,111001}. Is this subsequence infinite? - _Zak Seidov_, Jul 07 2015 %H A118688 Zak Seidov, Table of n, a(n) for n = 1..10000 %e A118688 55 is in the sequence because (1) it is a semiprime and (2) the sum of its digits 5+5=10 is also a semiprime. %p A118688 select(t -> map(numtheory:-bigomega,[t,convert(convert(t,base,10),`+`)])=[2,2], [$1..1000]); # _Robert Israel_, Jul 07 2015 %t A118688 Select[Range[514],PrimeOmega[{Total[IntegerDigits[#]],#}]=={2,2}&] (* _Zak Seidov_, Jul 07 2015 *) %o A118688 (PARI) A007953(n)= { local(resul); resul=0; while(n>0, resul += n%10; n = (n-n%10)/10; ); return(resul); } { for(n=4,600, if( bigomega(n)==2, if(bigomega(A007953(n)) == 2, print1(n,","); ); ); ); } \\ _R. J. Mathar_, May 23 2006 %Y A118688 Cf. A001358, A007953. %K A118688 base,nonn %O A118688 1,1 %A A118688 Luc Stevens (lms022(AT)yahoo.com), May 20 2006 %E A118688 Corrected by _R. J. Mathar_, May 23 2006 # Content is available under The OEIS End-User License Agreement: http://oeis.org/LICENSE