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A368249 a(n) = A002378(A005117(n)-1).

%I A368249 #11 Dec 23 2024 01:50:42
%S A368249 0,2,6,20,30,42,90,110,156,182,210,272,342,420,462,506,650,812,870,
%T A368249 930,1056,1122,1190,1332,1406,1482,1640,1722,1806,2070,2162,2550,2756,
%U A368249 2970,3192,3306,3422,3660,3782,4160,4290,4422,4692,4830,4970,5256,5402,5852,6006
%N A368249 a(n) = A002378(A005117(n)-1).
%C A368249 The squarefree oblong numbers (A229882) are all terms of this sequence, and their relative asymptotic density in it is A065474/A059956 = 0.530711... (A065469).
%H A368249 Amiram Eldar, <a href="/A368249/b368249.txt">Table of n, a(n) for n = 1..10000</a>
%F A368249 Sum_{n>=2} 1/a(n) = Sum_{k>=2} (zeta(k)/zeta(2*k) - 1) = 0.848633... (A368250).
%t A368249 Table[n*(n - 1), {n, Select[Range[100], SquareFreeQ]}]
%o A368249 (PARI) lista(kmax) = forsquarefree(k=1, kmax, print1(k[1]*(k[1]-1), ", "));
%o A368249 (Python)
%o A368249 from math import isqrt
%o A368249 from sympy import mobius
%o A368249 def A368249(n):
%o A368249     def f(x): return int(n-sum(mobius(k)*(x//k**2) for k in range(2, isqrt(x)+1)))
%o A368249     m, k = n, f(n)
%o A368249     while m != k: m, k = k, f(k)
%o A368249     return m*(m-1) # _Chai Wah Wu_, Dec 23 2024
%Y A368249 Cf. A002378, A005117, A229882.
%Y A368249 Cf. A059956, A065469, A065474, A368250.
%K A368249 nonn,easy
%O A368249 1,2
%A A368249 _Amiram Eldar_, Dec 19 2023