A131605 - OEIS

A131605

Perfect powers of nonprimes (m^k where m is a nonprime positive integer and k >= 2).

1, 36, 100, 144, 196, 216, 225, 324, 400, 441, 484, 576, 676, 784, 900, 1000, 1089, 1156, 1225, 1296, 1444, 1521, 1600, 1728, 1764, 1936, 2025, 2116, 2304, 2500, 2601, 2704, 2744, 2916, 3025, 3136, 3249, 3364, 3375, 3600, 3844, 3969, 4225, 4356, 4624

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OFFSET

1,2

COMMENTS

Although 1 is a square, is a cube, and so on..., 1 is sometimes excluded from perfect powers since it is not a well-defined power of 1 (1 = 1^k for any k in [2, 3, 4, 5, ...])

From Michael De Vlieger, Aug 11 2025: (Start)

This sequence is A001597 \ A246547; i.e., perfect powers without proper prime powers.

Union of {1} with the intersection of A001597 and A126706, where A126706 is the sequence of numbers that are neither prime powers nor squarefree.

Union of {1} and A286708 \ A052486; i.e., powerful numbers that are not prime powers, without Achilles numbers, but including the empty product. (End)

LINKS

Michael De Vlieger, Table of n, a(n) for n = 1..10000 (Terms a(n), n = 1..1323 from Klaus Brockhaus, and n = 1324..8649 from Daniel Forgues.)

FORMULA

Sum_{n>=1} 1/a(n) = 1 + A072102 - A136141 = 1.10130769935514973882... . - Amiram Eldar, Aug 15 2025

MAPLE

isA131605 := proc(n) local F; F := map(t -> t[2], ifactors(n)[2]); n=1 or (nops(F) > 1 and igcd(op(F)) > 1) end: select(isA131605, [$1..5000]); # Peter Luschny, Oct 07 2025

MATHEMATICA

With[{nn = 2^20}, {1}~Join~Select[Union@ Flatten@ Table[a^2*b^3, {b, Surd[nn, 3]}, {a, Sqrt[nn/b^3]}], And[Length[#2] > 1, GCD @@ #2 > 1] & @@ {#, FactorInteger[#][[;; , -1]]} &] ] (* Michael De Vlieger, Aug 11 2025 *)

PROG

(PARI) isok(n) = if (n == 1, return (1), return (ispower(n, , &np) && (! isprime(np)))); \\ Michel Marcus, Jun 12 2013

(Python)

from sympy import mobius, integer_nthroot, primepi

def A131605(n):

def f(x): return int(n-2+x+sum(mobius(k)*((a:=integer_nthroot(x, k)[0])-1)+primepi(a) for k in range(2, x.bit_length())))

kmin, kmax = 1, 2

while f(kmax) >= kmax:

kmax <<= 1

while True:

kmid = kmax+kmin>>1

if f(kmid) < kmid:

kmax = kmid

else:

kmin = kmid

if kmax-kmin <= 1:

break

return kmax # Chai Wah Wu, Aug 14 2024

CROSSREFS

Cf. A000961, A001597, A024619, A025475, A052486, A072102, A126706, A136141, A246547, A286708.

Sequence in context: A250812 A072413 A377817 * A390823 A391756 A376119

Adjacent sequences: A131602 A131603 A131604 * A131606 A131607 A131608

KEYWORD

nonn

AUTHOR

Daniel Forgues, May 27 2008

STATUS

approved