Search: id:A066265
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%I A066265
%S A066265 0,3,34,299,2625,23378,210035,1904324,17427258,160788536,1493776443,
%T A066265 13959990342,131126017178,1237088048653,11715902308080
%N A066265 Number of semiprimes < 10^n.
%C A066265 Apart from the first nonzero term the sequence is identical to A036352. 
               - Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003
%H A066265 Eric Weisstein's World of Mathematics, <a href="http://mathworld.wolfram.com/
               Semiprime.html">Link to a section of The World of Mathematics.</a>
%H A066265 <a href="Sindx_Pri.html#primepop">Index entries for sequences related 
               to numbers of primes in various ranges</a>
%F A066265 (1/2)*( Pi(10^(n/2)) + Sum_{i=1..Pi(10^n)} Pi( (10^n-1)/P_i) ) = Sum_{i=1..Pi(sqrt(10^n))} 
               Pi( (10^n-1)/P_i ) - Binomial( Pi(sqrt(10^n)), 2) (from Robert G. 
               Wilson v (rgwv(AT)rgwv.com), May 16 2005)
%e A066265 Below 10 there are three semiprimes: 4 (2*2), 6 (2*3) and 9 (3*3).
%t A066265 f[n_] := Sum[ PrimePi[(10^n - 1)/Prime[i]], {i, PrimePi[ Sqrt[10^n]]}] 
               - Binomial[ PrimePi[ Sqrt[10^n]], 2]; Do[ Print[ f[n]], {n, 0, 14}] 
               (from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005)
%Y A066265 Cf. A001358, A064911, A072000.
%Y A066265 Cf. A036352 (identical starting from a(2)).
%Y A066265 Sequence in context: A141789 A121077 A024396 this_sequence A134491 A045727 
               A105713
%Y A066265 Adjacent sequences: A066262 A066263 A066264 this_sequence A066266 A066267 
               A066268
%K A066265 nonn
%O A066265 0,2
%A A066265 Patrick De Geest (pdg(AT)worldofnumbers.com), Dec 10 2001.
%E A066265 More terms from Hugo Pfoertner (hugo(AT)pfoertner.org), Jul 22 2003
%E A066265 a(14) from Robert G. Wilson v (rgwv(AT)rgwv.com), May 16 2005

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