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Numbers which can be expressed as the product of numbers made of only sevens.
7

%I #9 Dec 04 2017 19:02:38

%S 1,7,49,77,343,539,777,2401,3773,5439,5929,7777,16807,26411,38073,

%T 41503,54439,59829,77777,117649,184877,266511,290521,381073,418803,

%U 456533,544439,598829,603729,777777,823543,1294139,1865577,2033647,2667511,2931621

%N Numbers which can be expressed as the product of numbers made of only sevens.

%C 49 = 7 * 7; 539 = 7 * 77; 5439 = 7 * 777.

%t Module[{upto=3*10^6,sevs=Table[FromDigits[PadRight[{},n,7]],{n,7}]},Select[ Union[ Flatten[Table[Times@@@Tuples[sevs,i],{i,7}]]],#<=upto&]] (* _Harvey P. Dale_, Feb 10 2015 *)

%Y Cf. A084034, A161140, A161141, A161142, A161143, A161144, A161146, A161147.

%K nonn,base

%O 1,2

%A _Claudio Meller_, Jun 03 2009

%E More terms from _Claudio Meller_, Jun 06 2009