OFFSET
1,2
COMMENTS
In 1657 Fermat challenged the world to find such numbers. [Dickson, Vol. 1, p. 54]
If n is a term and n is not divisible by 7, then 7*n is a term. - Don Dechman (dondechman_2000(AT)yahoo.com), Mar 26 2008
REFERENCES
A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 9.
L. E. Dickson, History of the Theory of Numbers. Carnegie Institute Public. 256, Washington, DC, Vol. 1, 1919; Vol. 2, 1920; Vol. 3, 1923, see vol. 1, p. 54.
Julian Havil, Gamma, Exploring Euler's Constant, Princeton University Press, Princeton and Oxford, 2003, page 92.
I. Kaplansky, The challenges of Fermat, Wallis and Ozanam (and several related challenges): I. Fermat's first challenge, Preprint, 2002.
LINKS
Donovan Johnson, Table of n, a(n) for n = 1..40 (terms < 10^11)
Donovan Johnson, 73688 terms > 10^11
Eric Weisstein's World of Mathematics, Fermat's Divisor Problem.
MATHEMATICA
max = 10^11; primes = {5, 7, 11, 13, 17, 19, 23, 31, 41, 43, 47, 83, 89, 191, 193, 239, 307, 443, 463, 499, 557, 701, 743, 1087, 1487, 2309, 3583, 4373, 5087, 5507, 5807, 44179}; subs = Select[ Times @@@ Subsets[primes, 7], # < max &] // Sort; f[e2_, e3_, p_] := If[n = 2^e2*3^e3*p; IntegerQ[ Sqrt[ DivisorSigma[1, n^3]]], Print[{2^e2, 3^e3, p}]; Sow[n]]; r = Reap[ Scan[ ((f[0, 0, #]; f[0, 1, #]; f[0, 3, #]; f[1, 0, #]; f[1, 1, #]; f[1, 3, #]; f[3, 0, #]; f[3, 1, #]; f[3, 3, #])& ), subs]][[2, 1]]; Select[r, # < max &] // Union (* Jean-François Alcover, Sep 07 2012, after Donovan Johnson *)
PROG
(PARI) is(n)=issquare(sigma(n^3)) \\ Charles R Greathouse IV, Jun 20 2013
(Python)
from functools import reduce
from operator import mul
from sympy import factorint, integer_nthroot
A008849_list, n = [], 1
while n < 10**7:
fs = factorint(n)
if integer_nthroot(reduce(mul, ((p**(3*fs[p]+1)-1)//(p-1) for p in fs), 1), 2)[1]:
A008849_list.append(n)
n += 1 # Chai Wah Wu, Apr 05 2021
CROSSREFS
KEYWORD
nonn,nice
AUTHOR
EXTENSIONS
David W. Wilson has supplied terms a(4) = 4730879 and beyond and verified completeness up to a(3) = 751530
I. Kaplansky and Will Jagy have verified that there are no other terms below 3.8*10^9
3656866255 added by Don Dechman, Mar 26 2008
STATUS
approved