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 A072490 #14 Aug 19 2024 11:38:35
%S A072490 0,0,1,2,2,3,4,5,5,5,6,7,7,8,9,10,10,11,11,12,12,13,14,15,15,15,16,16,
%T A072490 16,17,18,19,19,20,21,22,22,23,24,25,25,26,27,28,28,28,29,30,30,30,30,
%U A072490 31,31,32,32,33,33,34,35,36,36,37,38,38,38,39
%N A072490 Number of squarefree numbers (excluding 1) less than n.
%F A072490 a(n) = A013928(n) - 1, n > 1.
%F A072490 G.f.: (x/(1 - x)) * Sum_{k>=2} mu(k)^2*x^k. - _Ilya Gutkovskiy_, Sep 14 2019
%e A072490 a(10) = 5 as the squarefree numbers less than 10 are 2,3,5,6 and 7.
%o A072490 (PARI) a(n) = sum(k=2, n-1, issquarefree(k)); \\ _Michel Marcus_, Sep 15 2019
%o A072490 (Python)
%o A072490 from math import isqrt
%o A072490 from sympy import mobius
%o A072490 def A072490(n): return int(sum(mobius(k)*((n-1)//k**2) for k in range(1, isqrt(n-1)+1)))-1 if n>1 else 0 # _Chai Wah Wu_, Aug 19 2024
%Y A072490 Cf. A005117, A013928, A144338.
%K A072490 nonn
%O A072490 1,4
%A A072490 _Amarnath Murthy_, Jul 14 2002
%E A072490 Name clarified and more terms from _Ilya Gutkovskiy_, Sep 14 2019