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 A203967 #22 Feb 22 2025 14:49:39
%S A203967 0,1,2,3,4,4,5,5,6,6,7,7,8,8,8,9,10,10,11,11,11,11,12,12,13,13,13,13,
%T A203967 14,14,15,15,15,15,15,15,16,16,16,16,17,17,18,18,18,18,19,19,20,20,20,
%U A203967 20,21,21,21,21,21,21,22,22,23,23,23,24,24,24,25,25,25
%N A203967 The number of positive integers <= n that have a prime number of divisors.
%H A203967 Reinhard Zumkeller, <a href="/A203967/b203967.txt">Table of n, a(n) for n = 1..10000</a>
%F A203967 a(n) = a(n-1) + A010055(n) * A010051(A100995(n)+1). - _Reinhard Zumkeller_, Jun 06 2013
%t A203967 Table[Total[Table[PrimePi[m^(1/n)], {n,Table[Prime[n]-1, {n,1,20}]}]], {m,1,100}]
%t A203967 tot = 0; Table[If[PrimeQ[DivisorSigma[0, n]], tot++]; tot, {n, 100}] (* _T. D. Noe_, Jan 10 2012 *)
%o A203967 (Haskell)
%o A203967 a203967 n = length $ takeWhile (<= n) a009087_list
%o A203967 -- _Reinhard Zumkeller_, Jun 06 2013
%o A203967 (Python)
%o A203967 from sympy import primepi, integer_nthroot, primerange
%o A203967 def A203967(n): return int(sum(primepi(integer_nthroot(n,k-1)[0]) for k in primerange(n.bit_length()+1))) # _Chai Wah Wu_, Feb 22 2025
%Y A203967 Cf. A009087, A010051, A010055, A100995.
%K A203967 nonn
%O A203967 1,3
%A A203967 _Geoffrey Critzer_, Jan 08 2012