cp's OEIS Frontend

A108873 Numbers n whose base 3 representations, interpreted as base 10 integers, are semiprimes.

%I A108873 #12 Oct 13 2019 09:51:45
%S A108873 3,7,8,13,16,17,19,20,25,31,37,40,43,47,49,61,64,65,71,73,82,88,89,92,
%T A108873 97,100,101,106,110,115,118,121,127,136,142,143,155,179,184,187,188,
%U A108873 191,209,232,235,244,250,254,259,262,263,265,269,274,281,289,299,314,319
%N A108873 Numbers n whose base 3 representations, interpreted as base 10 integers, are semiprimes.
%e A108873 a(1) = 3 because 3 (base 10) = 10 (base 3) and 10 base 10 = 2 * 5.
%e A108873 a(2) = 7 because 7 (base 10) = 21 (base 3) and 21 base 10 = 3 * 7.
%e A108873 a(4) = 13 because 13 (base 10) = 111 (base 3) and 111 base 10 = 3 * 37.
%e A108873 a(12) = 40 because 40 (base 10) = 1111 (base 3) and 1111 base 10 = 11 * 101.
%e A108873 a(21) = 82 because 82 (base 10) = 10001 (base 3) and 10001 base 10 = 73 * 137.
%e A108873 a(26) = 100 because 100 (base 10) = 10201 (base 3) and 10201 base 10 = 101^2.
%p A108873 with(numtheory): a:=proc(n) local nn, nnn: nn:=convert(n,base,3): nnn:=add(nn[j]*10^(j-1),j=1..nops(nn)): if bigomega(nnn)=2 then n else fi end: seq(a(n),n=1..350); # _Emeric Deutsch_, Jul 16 2005
%t A108873 Select[Range[319], Plus @@ Last /@ FactorInteger[FromDigits[IntegerDigits[ #, 3]]] == 2 &] (* _Ray Chandler_, Sep 21 2005*)
%Y A108873 Cf. A001358, A007089.
%Y A108873 Cf. A110602 (base 4), A110603 (base 5), A110604 (base 6), A110605 (base 7) , A110606 (base 8), A110607 (base 9).
%K A108873 base,easy,nonn
%O A108873 1,1
%A A108873 _Jonathan Vos Post_, Jul 13 2005
%E A108873 More terms from _Emeric Deutsch_, Jul 16 2005