Search: id:A109251
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%I A109251
%S A109251 0,1,22,247,2569,25556,250853,2444359,23727305,229924367,2227121996,
%T A109251 21578747909,209214982913,2030133769624,19717814526785
%N A109251 Number of numbers up to 10^n which are products of three primes.
%F A109251 a(n)=A072114(10^n). - R. J. Mathar (mathar(AT)strw.leidenuniv.nl), May 
               25 2008
%e A109251 There are 22 numbers with three prime factors up to 10^2: 8, 12, 18, 
               20, 27, 28, 30, 42, 44, 45, 50, 52, 63, 66, 68, 70, 75, 76, 78, 92, 
               98, 99.
%t A109251 ThreeAlmostPrimePi[n_] := Sum[ PrimePi[n/(Prime@i*Prime@j)] - j + 1, 
               {i, PrimePi[n^(1/3)]}, {j, i, PrimePi@ Sqrt[n/Prime@i]}]; Table[ 
               ThreeAlmostPrimePi[10^n], {n, 0, 14}] (* Robert G. Wilson v *)
%Y A109251 Cf. A014612 = numbers with three prime factors, A036352 = number of numbers 
               up to 10^n which are products of two primes.
%Y A109251 Sequence in context: A035706 A041930 A125410 this_sequence A072076 A028571 
               A010974
%Y A109251 Adjacent sequences: A109248 A109249 A109250 this_sequence A109252 A109253 
               A109254
%K A109251 more,nonn
%O A109251 0,3
%A A109251 Martin Raab (raab-martin(AT)gmx.de), Aug 19 2005
%E A109251 a(10)-a(14) from Robert G. Wilson v (rgwv(at)rgwv.com), Feb 06 2006

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