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A006532 Numbers n such that sum of divisors of n is a square.
(Formerly M3089)
33
1, 3, 22, 66, 70, 81, 94, 115, 119, 170, 210, 214, 217, 265, 282, 310, 322, 343, 345, 357, 364, 382, 385, 400, 472, 497, 510, 517, 527, 642, 651, 679, 710, 742, 745, 782, 795, 820, 862, 884, 889, 930, 935, 966, 970, 1004, 1029, 1066, 1080, 1092, 1146 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

If a and b are in the sequence and relatively prime, then a*b is also in the sequence. - Franklin T. Adams-Watters, Jan 12 2009

Apart from a(2), all terms are composite. Bunyakovsky's conjecture implies that this sequence is infinite, since then (e.g.) there are infinitely many primes of the form p = 3k^2 - 1, whence sigma(2p) = 3p + 3 = 9k^2. - Charles R Greathouse IV, May 12 2011

REFERENCES

A. H. Beiler, Recreations in the Theory of Numbers, Dover, NY, 1964, p. 8.

J.-M. De Koninck, Ces nombres qui nous fascinent, Entry 94, p. 33, Ellipses, Paris 2008.

N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).

Abraham Verghese, Cutting for Stone: A Novel. New York: Alfred A. Knopf, 2009, p.361, p. 528 large-print edition.

David Wells, Curious and interesting numbers, Penguin Books, p. 111.

LINKS

T. D. Noe, Table of n, a(n) for n = 1..10000

Frits Beukers, Florian Luca and Frans Oort, Power Values of Divisor Sums, The American Mathematical Monthly, Vol. 119, No. 5 (May 2012), pp. 373-380.

Index entries for sequences related to sums of divisors

FORMULA

A010052(A000203(a(n))) = 1. - Reinhard Zumkeller, Jun 09 2013

EXAMPLE

3 is in the sequence because its divisors are 1 and 3, which add up to 4 = 2^2.

22 is in the sequence because its divisors are 1, 2, 11, 22, which add up to 36 = 6^2.

32 is not in the sequence, because its divisors, 1, 2, 4, 8, 16, 32, add up to 63, which is one short of 8^2.

MAPLE

for i from 1 to 1000 do if issqr(sigma(i)) then print(i); fi; od;

MATHEMATICA

Select[ Range[ 1150 ], IntegerQ[ Sqrt[ DivisorSigma[ 1, # ] ] ]& ]

PROG

(PARI) is(n)=issquare(sigma(n)) \\ Charles R Greathouse IV, Jun 05 2013

(Haskell)

a006532 n = a006532_list !! (n-1)

a006532_list = filter ((== 1) . a010052 . a000203) [1..]

-- Reinhard Zumkeller, Jun 09 2013

(Sage) [n for n in (1..1000) if sigma(n).is_square()] # Giuseppe Coppoletta, Dec 16 2014

(MAGMA) [n: n in [1..2000] | IsSquare(&+(Divisors(n)))]; // Vincenzo Librandi, May 31 2015

CROSSREFS

Cf. A074385, A000203, A020477.

Sequence in context: A104604 A139272 A159345 * A274870 A178492 A005288

Adjacent sequences:  A006529 A006530 A006531 * A006533 A006534 A006535

KEYWORD

nonn,nice

AUTHOR

N. J. A. Sloane, Simon Plouffe

EXTENSIONS

a(42)-a(51) from Enoch Haga, circa 1999

STATUS

approved

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Last modified March 29 06:34 EDT 2017. Contains 284250 sequences.