A340588 -id:A340588 - cp's OEIS frontend

A374291 Squares of powerful numbers.

1, 16, 64, 81, 256, 625, 729, 1024, 1296, 2401, 4096, 5184, 6561, 10000, 11664, 14641, 15625, 16384, 20736, 28561, 38416, 40000, 46656, 50625, 59049, 65536, 82944, 83521, 104976, 117649, 130321, 153664, 160000, 186624, 194481, 234256, 250000, 262144, 279841, 331776

Offset: 1

Amiram Eldar, Jul 02 2024

First differs from A340588 at n = 12.
4-full (or 3-full) squares.
Numbers whose exponents in their prime factorization are all even numbers >= 4.
This sequence is closed under multiplication.
The sequence {A000290(n)*A078615(A000290(n)), n>=1} is a permutation of this sequence, and the sequence {a(n)/A078615(a(n)), n>=1} is a permutation of {A000290(n), n>=1}.
The sequence {A335988(n)*A007947(A335988(n)), n>=1} is a permutation of this sequence, and the sequence {a(n)/A007947(a(n)), n>=1} is a permutation of A335988.

Amiram Eldar, Table of n, a(n) for n = 1..10000
Index entries for sequences related to powerful numbers.

Intersection of A000290 and A036967 (or A036966).
Intersection of A000290 and A337050.
Subsequence of A322449.
Cf. A000290, A001694, A007947, A078615, A335988, A340588.

powQ[n_] := n==1 || AllTrue[FactorInteger[n][[;; , 2]], # > 1 &]; Select[Range[600], powQ]^2

is(k) = issquare(k) && ispowerful(sqrtint(k));

from math import isqrt
from sympy import mobius, integer_nthroot
def A374291(n):
    def squarefreepi(n):
        return int(sum(mobius(k)*(n//k**2) for k in range(1, isqrt(n)+1)))
    def bisection(f, kmin=0, kmax=1):
        while f(kmax) > kmax: kmax <<= 1
        while kmax-kmin > 1:
            kmid = kmax+kmin>>1
            if f(kmid) <= kmid:
                kmax = kmid
            else:
                kmin = kmid
        return kmax
    def f(x):
        c, l = n+x, 0
        j = isqrt(x)
        while j>1:
            k2 = integer_nthroot(x//j**2, 3)[0]+1
            w = squarefreepi(k2-1)
            c -= j*(w-l)
            l, j = w, isqrt(x//k2**3)
        c -= squarefreepi(integer_nthroot(x, 3)[0])-l
        return c
    return bisection(f,n,n)**2 # Chai Wah Wu, Sep 10 2024

a(n) = A000290(A001694(n)) = A001694(n)^2.
Sum_{n>=1} 1/a(n) = Product_{p prime} (1 + 1/(p^2*(p^2-1))) = zeta(4)*zeta(6)/zeta(12) = 15015/(1382*Pi^2) = 1.10082313486953808844... .
Sum_{n>=1} 1/a(n)^s =  Product_{p prime} (1 + 1/(p^(2*s)*(p^(2*s)-1))) = zeta(4*s)*zeta(6*s)/zeta(12*s), for s > 1/4.

A335590 Decimal expansion of the sum of the reciprocals of the squares of the perfect powers > 1.

Original entry on oeis.org

1, 0, 0, 4, 7, 5, 3, 2, 7, 2, 0, 0, 0, 9, 3, 7, 7, 5, 8, 6, 0, 1, 4, 8, 9, 5, 1, 6, 4, 3, 6, 7, 9, 5, 0, 3, 8, 9, 3, 0, 2, 8, 8, 3, 9, 9, 2, 4, 7, 2, 4, 4, 8, 9, 4, 5, 6, 1, 9, 2, 9, 4, 0, 6, 1, 0, 6, 3, 5, 7, 7, 3, 4, 9, 4, 4, 6, 9, 2, 1, 7, 0, 5, 0, 9, 5, 8, 5, 2, 0, 5, 1, 2, 1, 8, 1, 6, 3, 9, 7, 6, 2, 0, 5, 7

Offset: 0

Jon E. Schoenfield, Jan 26 2021

			Equals 1/4^2 + 1/8^2 + 1/9^2 + 1/16^2 + 1/25^2 + 1/27^2 + 1/32^2 + 1/36^2 + 1/49^2 + 1/64^2 + 1/81^2 + 1/100^2 + ... = 0.10047532720009377586014895164367950389302883992472...

Cf. A001597, A013661, A085548, A275647, A335086, A335589, A340588.

RealDigits[Sum[MoebiusMu[k]*(1 - Zeta[2*k]), {k, 2, 200}], 10, 105][[1]] (* Amiram Eldar, Jan 27 2021 *)

suminf(k=2,moebius(k)*(1-zeta(2*k))) \\ Hugo Pfoertner, Jan 27 2021

Equals Sum_{k>=2} 1/A001597(k)^2.

Equals Sum_{k>=2} mu(k)*(1 - zeta(2*k)). - Amiram Eldar, Jan 27 2021

cp's OEIS Frontend

A374291 Squares of powerful numbers.

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