A118690 - OEIS

%I #13 Mar 07 2024 08:52:23

%S 4,6,9,22,33,55,77,91,123,141,177,213,217,321,411,717,721,771,1135,

%T 1157,1315,1351,1513,1517,1751,3151,5131,5311,7115,11157,11215,11521,

%U 11611,11751,12115,12151,15117,15711,17151,21151,51211,57111,61111,75111

%N Semiprimes for which both the sum and the product of the digits is also a semiprime.

%H Vincenzo Librandi, <a href="/A118690/b118690.txt">Table of n, a(n) for n = 1..128</a>

%e 91 is in the sequence because (1) it is a semiprime, (2) the sum of its digits is 9+1=10, (3) the product of its digits is 1*9=9 and (4) both 9 and 10 are semiprimes.

%t spQ[n_] := Plus @@ Last /@ FactorInteger@n == 2; fQ[n_] := Block[{id = IntegerDigits@n}, spQ@n && spQ[Plus @@ id] && spQ[Times @@ id]]; Select[ Range@111172, fQ@# &] (* _Robert G. Wilson v_, Jun 15 2006 *)

%t Select[Range[76000],Thread[PrimeOmega[{#,Total[IntegerDigits[#]],Times@@ IntegerDigits[#]}]] == {2,2,2}&]  (* _Harvey P. Dale_, Jul 26 2022 *)

%Y Cf. A001358.

%K base,nonn

%O 1,1

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

%E More terms from _Robert G. Wilson v_, Jun 15 2006