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 A362042 #23 Apr 16 2023 21:01:01
%S A362042 0,0,0,0,2,4,11,24,51,103,207,417,815,1622,3164,6210,12146,23711,
%T A362042 46295,90307,176369,344155,672091,1312721,2565048,5013566,9804910,
%U A362042 19183069,37547164,73526846,144042323,282317826,553564500,1085869406,2130916524,4183381508,8215884036
%N A362042 Number of odd semiprimes less than 2^n.
%C A362042 Odd numbers with two prime factors are used as the modulus in the RSA algorithm. This sequence shows the growth of the number of 'candidate' RSA moduli for keys up to a given number of bits.
%F A362042 a(n) = A125527(n) - A007053(n-1) for n > 0. - _Jinyuan Wang_, Apr 16 2023
%e A362042 For n=5, there are four integers less than 32 (i.e., 2^5) that are the product of two odd primes: {3*3, 3*5, 3*7, 5*5} = {9, 15, 21, 25}; hence, a(5)=4.
%t A362042 a[n_]:=Length@Select[Range[1, 2^n - 1, 2], Total[Last /@ FactorInteger[#]] == 2 &]
%t A362042 Table[a[n],{n,0,24}]
%Y A362042 Cf. A046315, A007053, A085770, A125527.
%K A362042 nonn
%O A362042 0,5
%A A362042 _Sidney Cadot_, Apr 15 2023
%E A362042 More terms from _Jinyuan Wang_, Apr 16 2023