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A072188 Numbers k that divide sigma(k-1)+sigma(k)+sigma(k+1), where sigma() is the "sum of integer divisors" function. 3
2, 5, 7, 33, 18336, 19262, 38184, 54722, 458010, 1402224, 1473624, 1708480, 3680560, 3733584, 5829841, 7769904, 13204262, 72873362, 136910448, 252961224, 509874800, 1138081558, 3414097921, 5440847376, 7285326504, 8986032648, 14409659936, 15953956256, 23556877704 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Heuristics would seem to indicate that this sequence is infinite and sparse. Compare this sequence to the positive integers k that divide sigma(k) - the perfect and multiperfect numbers.

LINKS

Giovanni Resta, Table of n, a(n) for n = 1..36 (terms < 10^13)

EXAMPLE

33 is in this list because sigma(32) + sigma(33) + sigma(34) = 63 + 48 + 54 = 165 = 33*5.

PROG

(PARI) for(i=2, 99999, if((sigma(i-1)+sigma(i)+sigma(i+1))%i==0, print(i)))

CROSSREFS

Cf. A007691.

Sequence in context: A042931 A085394 A041875 * A277056 A160621 A028432

Adjacent sequences:  A072185 A072186 A072187 * A072189 A072190 A072191

KEYWORD

nonn

AUTHOR

Jack Brennen, Jul 01 2002

EXTENSIONS

More terms from Jason Earls, Aug 25 2002

a(18)-a(26) from Donovan Johnson, Feb 01 2009

a(27)-a(29) from Donovan Johnson, Sep 17 2013

STATUS

approved

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Last modified October 18 23:35 EDT 2021. Contains 348071 sequences. (Running on oeis4.)