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A118688
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Semiprimes for which the sum of the digits is also a semiprime.
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4
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4, 6, 9, 15, 22, 33, 46, 51, 55, 69, 77, 82, 86, 87, 91, 95, 118, 121, 123, 141, 145, 158, 159, 177, 185, 194, 202, 213, 217, 226, 235, 249, 253, 262, 267, 301, 303, 321, 329, 334, 339, 361, 365, 393, 411, 415, 437, 446, 447, 451, 473, 482, 489, 501, 505, 514
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OFFSET
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1,1
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COMMENTS
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The first term congruent to 2 mod 9 is a(2729) = 29999. - Robert Israel, Jul 07 2015
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
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LINKS
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EXAMPLE
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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.
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MAPLE
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select(t -> map(numtheory:-bigomega, [t, convert(convert(t, base, 10), `+`)])=[2, 2], [$1..1000]); # Robert Israel, Jul 07 2015
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MATHEMATICA
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Select[Range[514], PrimeOmega[{Total[IntegerDigits[#]], #}]=={2, 2}&] (* Zak Seidov, Jul 07 2015 *)
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PROG
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(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
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CROSSREFS
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KEYWORD
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base,nonn
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AUTHOR
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Luc Stevens (lms022(AT)yahoo.com), May 20 2006
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EXTENSIONS
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STATUS
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approved
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