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%I #23 Apr 16 2023 21:01:01
%S 0,0,0,0,2,4,11,24,51,103,207,417,815,1622,3164,6210,12146,23711,
%T 46295,90307,176369,344155,672091,1312721,2565048,5013566,9804910,
%U 19183069,37547164,73526846,144042323,282317826,553564500,1085869406,2130916524,4183381508,8215884036
%N Number of odd semiprimes less than 2^n.
%C 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 a(n) = A125527(n) - A007053(n-1) for n > 0. - _Jinyuan Wang_, Apr 16 2023
%e 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 a[n_]:=Length@Select[Range[1, 2^n - 1, 2], Total[Last /@ FactorInteger[#]] == 2 &]
%t Table[a[n],{n,0,24}]
%Y Cf. A046315, A007053, A085770, A125527.
%K nonn
%O 0,5
%A _Sidney Cadot_, Apr 15 2023
%E More terms from _Jinyuan Wang_, Apr 16 2023
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