A175066 a(1) = 1, for n >= 2: a(n) = number of ways h to write perfect powers A117453(n) as m^k (m >= 2, k >= 2).

1, 2, 3, 2, 3, 2, 2, 3, 3, 2, 2, 5, 3, 2, 2, 3, 3, 2, 2, 2, 3, 2, 3, 2, 3, 4, 2, 2, 3, 2, 2, 2, 2, 5, 2, 2, 3, 2, 5, 2, 2, 2, 2, 3, 5, 2, 2, 2, 3, 3, 2, 2, 2, 3, 2, 2, 2, 3, 2, 2, 2, 2, 3, 2, 3, 3, 2, 2, 3, 2, 2, 2, 3, 2, 2, 2, 3, 2, 2, 2, 2, 3, 2, 2, 2, 3, 2, 2, 7, 2, 2, 2, 2, 2, 2, 3, 2, 2, 2, 2, 2, 2, 2, 3, 2

Offset

1,2

Comments

Perfect powers with first occurrence of h >= 2: 16, 64, 65536, 4096, ... (A175065)

a(n) for n>1 is the subsequence of A253642 formed by the terms which exceed 1; equivalently, a(n)+1 for n>1 is the subsequence of A175064 formed by the terms which exceed 2. Also, sum of a(n)-1 over such n that A117453(n)<10^m gives A275358(m). - Andrey Zabolotskiy, Aug 16 2016

Numbers n such that a(n) is nonprime are 1, 26, 110, ... - Altug Alkan, Aug 22 2016

Links

Table of n, a(n) for n=1..105.

Formula

If A117453(n) = m^k with k maximal, then a(n) = tau(k) - 1. - Charlie Neder, Mar 02 2019

Example

For n = 12, A117453(12) = 4096 and a(12)=5 since there are 5 ways to write 4096 as m^k: 64^2 = 16^3 = 8^4 = 4^6 = 2^12.
729=27^2=9^3=3^6 and 1024=32^2=4^5=2^10 yield a(8)=a(9)=3. - R. J. Mathar, Jan 24 2010

Prog

(PARI) lista(nn) = {print1(1, ", "); for (i=2, nn, if (po = ispower(i), np = sum(j=2, po, ispower(i, j)); if (np>1, print1(np, ", ")); ); ); } \\ Michel Marcus, Mar 20 2013
(Python)
from math import gcd
from sympy import mobius, integer_nthroot, factorint, divisor_count, primerange
def A175066(n):
    if n == 1: return 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): return int(n+sum(mobius(k)*(integer_nthroot(x, k)[0]-1+sum(integer_nthroot(x, p*k)[0]-1 for p in primerange((x//k).bit_length()))) for k in range(1, x.bit_length())))
    return divisor_count(gcd(*factorint(bisection(f, n, n)).values()))-1 # Chai Wah Wu, Nov 24 2024

Crossrefs

Cf. A117453.

Sequence in context: A296658 A276862 A371442 * A066102 A036048 A145384

Adjacent sequences: A175063 A175064 A175065 * A175067 A175068 A175069

Keyword

nonn,changed

Author

Jaroslav Krizek, Jan 23 2010

Extensions

Corrected and extended by R. J. Mathar, Jan 24 2010

Status

approved