A099631 - OEIS

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A099631

Numbers n such that (sigma(n-2)+sigma(n+2))/2 = sigma(n).

5, 8, 32, 44, 116, 184, 356, 764, 815, 1004, 1012, 1205, 1338, 1345, 1436, 2636, 2876, 3126, 3644, 5156, 5305, 5804, 6236, 7192, 7542, 7556, 8564, 9356, 9404, 10196, 10362, 10904, 11276, 11876, 13436, 16375, 17564, 17636, 20156, 25316, 25964

(list; graph; refs; listen; history; text; internal format)

OFFSET

1,1

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

n=5, {n-2,n+2}={3,7}, sigma(3)=4, sigma(7)=8, and sigma(5)=6=(4+8)/2.

MAPLE

with(numtheory); A099631:=n->`if`(sigma(n)=(sigma(n-2)+sigma(n+2))/2, n, NULL); # Wesley Ivan Hurt, Feb 03 2014

MATHEMATICA

Flatten[Position[Partition[DivisorSigma[1, Range[30000]], 5, 1], _?((Last[ #]+ First[ #])/2==#[[3]]&), {1}, Heads->False]]+2 (* Harvey P. Dale, Feb 01 2015 *)

PROG

(PARI) isok(n) = (sigma(n-2) + sigma(n+2))/2 == sigma(n); \\ Michel Marcus, Dec 30 2013

CROSSREFS

Cf. A000203.

Sequence in context: A304647 A002536 A068981 * A199396 A275003 A032790

Adjacent sequences: A099628 A099629 A099630 * A099632 A099633 A099634

KEYWORD

nonn

AUTHOR

Labos Elemer, Oct 26 2004

STATUS

approved