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A008849
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Numbers n such that the sum of divisors of n^3 is a square.
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5
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1, 7, 751530, 4730879, 5260710, 33116153, 37200735, 187062910, 226141311, 259109835, 260405145, 370049418, 522409465, 836308083, 1105725765, 1309440370, 1343713507, 1582989177, 1609505430, 1813768845, 2590345926, 3039492538, 3656866255
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OFFSET
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1,2
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COMMENTS
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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
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REFERENCES
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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.
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LINKS
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MATHEMATICA
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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 *)
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PROG
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(Python)
from functools import reduce
from operator import mul
from sympy import factorint, integer_nthroot
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]:
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CROSSREFS
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KEYWORD
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nonn,nice
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AUTHOR
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EXTENSIONS
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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
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STATUS
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approved
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