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Number of semiprimes with n digits.
0

%I #22 Feb 10 2022 22:53:49

%S 3,31,265,2326,20753,186657,1694289,15522934,143361278,1332987907,

%T 12466213899,117166026836,1105962031475,10478814259427,99613914990801,

%U 949727475528388,9078425620890083,86983554771459735,835176988544997091,8034305823411044844,77423350444120266442

%N Number of semiprimes with n digits.

%F a(n) = A066265(n) - A066265(n-1). - _Jens Kruse Andersen_, Aug 16 2014

%e 4, 6, and 9 are semiprimes with 1 digit. So a(1) = 3.

%o (PARI) vector(8,n,sum(k=10^(n-1),10^n-1,bigomega(k)==2))

%Y Cf. A001358, A066265.

%K nonn,hard,base

%O 1,1

%A _Derek Orr_, Aug 15 2014

%E a(9)-a(18) from A066265, added by _Jens Kruse Andersen_, Aug 16 2014

%E a(19)-a(21) from A066265, added by _Jon E. Schoenfield_, Feb 10 2022