%I #8 Oct 12 2019 12:00:25

%S 4,6,7,11,12,15,16,19,20,24,25,26,34,36,40,44,47,55,57,61,64,65,66,73,

%T 74,78,79,80,82,83,89,92,97,99,100,101,103,104,108,109,110,113,118,

%U 124,136,137,142,145,148,149,150,152,158,162,164,167,172,173,176,181,187

%N Numbers n whose base 7 representations, interpreted as base 10 integers, are semiprimes.

%C A108873 is the equivalent using base 3. A110602 is the equivalent using base 4. A110603 is the equivalent using base 5. A110604 is the equivalent using base 6. A110606 is the equivalent using base 8. A110607 is the equivalent using base 9.

%e a(1) = 4 because 4 (base 7) = 4 (base 10) = 2 * 2, a semiprime (A001358).

%e a(2) = 6 because 6 (base 7) = 6 (base 10) = 2 * 3

%e a(3) = 7 because 7 (base 7) = 10 and 10 (base 10) = 2 * 5.

%e a(4) = 11 because 11 (base 7) = 14 and 14 (base 10) = 2 * 7.

%e a(5) = 12 because 12 (base 7) = 15 and 15 (base 10) = 3 * 5.

%t Select[Range[187], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 7]]] == 2 &] (* _Ray Chandler_, Aug 05 2005 *)

%Y Cf. A001358, A007093, A108873, A110602, A110603, A110604, A110606, A110607.

%K base,easy,nonn

%O 1,1

%A _Jonathan Vos Post_, Jul 30 2005

%E Extended by _Ray Chandler_, Aug 05 2005