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%I #23 Jul 31 2021 02:00:58
%S 0,3,15,95,669,5133,41538,348513,3001134,26355867,234954223,
%T 2119654578,19308136142,177291661649,1638923764567,15237833654620,
%U 142377417196364,1336094767763971,12585956566571620,118959989688273472,1127779923790184543,10720710117789005897
%N Number of even semiprimes < 10^n. Number of terms of A100484 < 10^n.
%C All such semiprimes have the form 2*p, where p is prime. - _T. D. Noe_, Dec 09 2012
%H Kim Walisch, <a href="https://github.com/kimwalisch/primecount">Fast C++ prime counting function implementation</a>.
%F a(n) = A066265(n) - A085770(n) for n > 1.
%t Table[PrimePi[10^n/2], {n, 0, 14}]
%o (PARI) a(n)=primepi(10^n\2) \\ _Charles R Greathouse IV_, Sep 08 2015
%Y Cf. A066265, A085770, A100484.
%K nonn
%O 0,2
%A _Robert G. Wilson v_, Dec 08 2012
%E a(15)-a(20) from _Hugo Pfoertner_, Oct 14 2017
%E a(21) from _Jinyuan Wang_, Jul 30 2021
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