A346676 - OEIS

%I #17 Jul 30 2021 08:29:59

%S 5,7,11,13,17,19,25,29,31,35,41,43,59,67,73,83,85,89,91,97,113,131,

%T 137,145,155,209,245,247,251,259,265,275,283,307,337,371,499,515,521,

%U 539,593,731,733,737,745,755,761,793,857,985,1027,1033,1051,1105,1241

%N Numbers expressible as 2^x + 3^y where both x and y are positive integers.

%C All terms have the form 6k +- 1.

%F { A004050 } minus { A000079, A000244 }.

%o (PARI) f(x,y) = 2^x + 3^y;

%o lista(nn) = select(x->(x<=nn), setbinop(f, [1..logint(nn, 2)], [1..logint(nn, 3)])); \\ _Michel Marcus_, Jul 29 2021

%o (Python)

%o def aupto(lim):

%o     s, pow3 = set(), 3

%o     while pow3 < lim:

%o         for j in range(1, (lim-pow3).bit_length()):

%o             s.add(2**j + pow3)

%o         pow3 *= 3

%o     return sorted(set(s))

%o print(aupto(1242)) # _Michael S. Branicky_, Jul 29 2021

%Y Cf. A000079, A000244, A004050.

%K nonn

%O 1,1

%A _Keith Backman_, Jul 28 2021