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 A146168 #26 Sep 06 2024 14:08:32
%S A146168 0,0,0,1,2,8,20,46,96,197,404,798,1599,3134,6169,12093,23640,46199,
%T A146168 90180,176198,343927,671783,1312304,2564485,5012807,9803883,19181677,
%U A146168 37545265,73524262,144038812,282313035,553557959,1085860455,2130904274,4183364732,8215861037
%N A146168 Number of odd squarefree semiprimes (A046388) < 2^n.
%H A146168 Amiram Eldar, <a href="/A146168/b146168.txt">Table of n, a(n) for n = 1..57</a> (calculated using the b-files at A060967, A007053 and A125527)
%F A146168 a(n) = A125527(n) - A060967(n) - A007053(n-1) + 1, for n > 1.
%e A146168 a(5) = 2. The odd squarefree semiprimes less than 2^5 are 15 and 21. The formula gives 10 - pi(5) - pi(2^4) + 1 = 2.
%t A146168 Table[lim=2^n; Sum[PrimePi[lim/p]-PrimePi[p], {p, Prime[Range[2,PrimePi[Sqrt[lim]]]]}], {n,20}]
%Y A146168 Cf. A046388, A001358 (semiprimes), A000720 (pi(n), the number of primes <= n), A007053 (number of primes <= 2^n), A060967, A125527 (number of semiprimes <= 2^n).
%K A146168 nonn
%O A146168 1,5
%A A146168 _Washington Bomfim_, Oct 27 2008
%E A146168 a(34) onwards from _Amiram Eldar_, Sep 05 2024