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Multiples of 9 which are 7-smooth.
7

%I #14 Sep 23 2024 08:38:42

%S 9,18,27,36,45,54,63,72,81,90,108,126,135,144,162,180,189,216,225,243,

%T 252,270,288,315,324,360,378,405,432,441,450,486,504,540,567,576,630,

%U 648,675,720,729,756,810,864,882,900,945,972,1008,1080,1125,1134,1152

%N Multiples of 9 which are 7-smooth.

%C Equivalently, multiples of 9 with the largest prime divisor < 10.

%F From _Amiram Eldar_, Sep 22 2024: (Start)

%F a(n) = 9*A002473(n).

%F Sum_{n>=1} 1/a(n) = 35/72. (End)

%t With[{p = Prime[Range[4]]}, 9 * Select[Range[140], Times @@ (p^IntegerExponent[#, p]) == # &]] (* _Amiram Eldar_, Sep 23 2024 *)

%Y Intersection of A008591 and A002473.

%Y Cf. A085125, A085126, A085127, A085128, A085129, A080194, A085131.

%K nonn,easy

%O 1,1

%A _Amarnath Murthy_, Jul 06 2003

%E More terms from _David Wasserman_, Jan 28 2005

%E Offset changed by _Andrew Howroyd_, Sep 19 2024