This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A220262 #27 Oct 17 2024 15:50:02
%S A220262 0,3,15,95,669,5133,41538,348513,3001134,26355867,234954223,
%T A220262 2119654578,19308136142,177291661649,1638923764567,15237833654620,
%U A220262 142377417196364,1336094767763971,12585956566571620,118959989688273472,1127779923790184543,10720710117789005897
%N A220262 Number of even semiprimes < 10^n. Number of terms of A100484 < 10^n.
%C A220262 All such semiprimes have the form 2*p, where p is prime. - _T. D. Noe_, Dec 09 2012
%H A220262 Kim Walisch, <a href="https://github.com/kimwalisch/primecount">Fast C++ prime counting function implementation</a>.
%F A220262 a(n) = A066265(n) - A085770(n) for n > 1.
%t A220262 Table[PrimePi[10^n/2], {n, 0, 14}]
%o A220262 (PARI) a(n)=primepi(10^n\2) \\ _Charles R Greathouse IV_, Sep 08 2015
%o A220262 (Python)
%o A220262 from sympy import primepi
%o A220262 def A220262(n): return primepi(10**n>>1) # _Chai Wah Wu_, Oct 17 2024
%Y A220262 Cf. A066265, A085770, A100484.
%K A220262 nonn
%O A220262 0,2
%A A220262 _Robert G. Wilson v_, Dec 08 2012
%E A220262 a(15)-a(20) from _Hugo Pfoertner_, Oct 14 2017
%E A220262 a(21) from _Jinyuan Wang_, Jul 30 2021