A377880 - OEIS

%I #23 Nov 14 2024 02:11:21

%S 1,4,6,9,10,12,14,15,18,20,21,22,25,26,28,30,33,34,35,36,38,39,42,44,

%T 45,46,49,50,51,52,55,57,58,60,62,63,65,66,68,69,70,74,75,76,77,78,82,

%U 84,85,86,87,90,91,92,93,94,95,98,99,100,102,105,106,110,111

%N Nonprime cubefree numbers.

%p q:= n-> not isprime(n) and andmap(i-> i[2]<3, ifactors(n)[2]):

%p select(q, [$1..111])[];  # _Alois P. Heinz_, Nov 10 2024

%t Select[Range[120], !PrimeQ[#] && Max[FactorInteger[#][[;; , 2]]] < 3 &] (* _Amiram Eldar_, Nov 10 2024 *)

%o (Python)

%o from sympy import primepi, mobius, integer_nthroot

%o def A377880(n):

%o     def f(x): return int(n+x+primepi(x)-sum(mobius(k)*(x//k**3) for k in range(1, integer_nthroot(x,3)[0]+1)))

%o     m, k = n, f(n)

%o     while m != k: m, k = k, f(k)

%o     return m # _Chai Wah Wu_, Nov 13 2024

%Y Intersection of A004709 and A018252.

%Y Cf. A000469.

%K nonn

%O 1,2

%A _Sahil K. Das_, Nov 10 2024