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A185445
Smallest number having exactly t divisors, where t is the n-th triprime (A014612).
1
24, 60, 180, 240, 900, 960, 720, 2880, 15360, 3600, 6480, 61440, 14400, 46080, 983040, 25920, 32400, 3932160, 184320, 62914560, 233280, 230400, 2949120, 129600, 414720, 11796480, 4026531840, 921600, 16106127360, 810000, 1658880, 188743680, 1166400, 1030792151040, 14745600, 3732480
OFFSET
1,1
COMMENTS
This is the 3rd row of an infinite array A[k,n] = smallest number having exactly j divisors where j is the n-th natural number with exactly k prime factors (with multiplicity).
The first row is A061286, the second row is A096932.
FORMULA
a(n) = A005179(A014612(n)).
EXAMPLE
a(10) is 3600 because the 10th triprime is 45, and the smallest number with exactly 45 factors is 3600 = 2^4 * 3^2 * 5^2.
a(20) is 62914560 because the 10th triprime is 92, and the smallest number with exactly 92 factors is 62914560 = 2^22 * 3 * 5.
PROG
(Python)
from math import isqrt, prod
from sympy import isprime, primepi, primerange, integer_nthroot, prime, divisors
def A185445(n):
def mult_factors(n):
if isprime(n):
return [(n, )]
c = []
for d in divisors(n, generator=True):
if 1<d<n:
for a in mult_factors(n//d):
c.append(tuple(sorted((d, )+a)))
return list(set(c))
def f(x): return int(n+x-sum(primepi(x//(k*m))-b for a, k in enumerate(primerange(integer_nthroot(x, 3)[0]+1)) for b, m in enumerate(primerange(k, isqrt(x//k)+1), a)))
m, k = n, f(n)
while m != k:
m, k = k, f(k)
return min((prod(prime(i)**(j-1) for i, j in enumerate(reversed(d), 1)) for d in mult_factors(m)), default=1) # Chai Wah Wu, Aug 17 2024
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Jonathan Vos Post, Feb 03 2011
STATUS
approved