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%I #8 Oct 12 2019 12:01:38
%S 4,9,10,15,21,25,26,27,31,33,34,35,39,41,49,50,51,57,59,67,69,81,85,
%T 89,95,98,99,101,119,122,127,137,141,143,145,146,147,154,155,161,173,
%U 178,185,187,199,202,205,209,211,215,221,223,226,235,239,257,259,265,266
%N Numbers n whose base 4 representations, interpreted as base 10 integers, are semiprimes.
%C Note that this sequence begins with 7 semiprimes (A001358) but is not a subset of the semiprimes, since a(9) = 31 and 31 (base 4) = 133, which interpreted as base 10 is 133 = 7 * 19, yet 31 (base 10) is prime rather than semiprime. A108873 is the equivalent using base 3. A110603 is the equivalent using base 5. A110604 is the equivalent using base 6. A110605 is the equivalent using base 7. A110606 is the equivalent using base 8. A110607 is the equivalent using base 9.
%e a(1) = 4 because 4 (base 4) = 10 and 10 (base 10) = 2 * 5, a semiprime (A001358).
%e a(2) = 9 because 10 (base 4) = 21, which interpreted base 10 is 21 = 3 * 7.
%e a(3) = 10 because 10 (base 4) = 22, which interpreted base 10 is 22 = 2 * 11.
%e a(4) = 15 because 15 (base 4) = 33, which interpreted base 10 is 33 = 3 * 11.
%t Select[Range[266], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 4]]] == 2 &] (* _Ray Chandler_, Aug 05 2005 *)
%Y Cf. A001358, A007090, A108873, A110603, A110604, A110605, A110606, A110607.
%K base,easy,nonn
%O 1,1
%A _Jonathan Vos Post_, Jul 30 2005
%E Extended by _Ray Chandler_, Aug 05 2005
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