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A067931 Numbers n such that n divides the alternating sum sigma(1)-sigma(2)+sigma(3)-sigma(4)+...+((-1)^(n+1))sigma(n). 2
1, 2, 11, 19, 36, 45, 152, 377, 418, 3794, 4423, 14495, 31148, 42224, 49279, 120447, 1018376, 2605261, 17484247, 368070997, 850833878, 1121254607, 3440701629, 7863041200 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,2

COMMENTS

No further term below 10^7.

a(25) > 5*10^10. - Donovan Johnson, Jul 26 2011

LINKS

Table of n, a(n) for n=1..24.

EXAMPLE

sigma(1)-sigma(2) = -2, which is divisible by 2, so 2 is a term of the sequence.

MATHEMATICA

s = 0; Do[s = s + (-1)^(i + 1) * DivisorSigma[1, i]; If[Mod[s, i] == 0, Print[i]], {i, 1, 10^5}]

PROG

(PARI) {a067931(m)=local(s, n); s=0; for(n=1, m, if(n%2==0, s=s-sigma(n), s=s+sigma(n)); if(s%n==0, print1(n, ", ")))}

CROSSREFS

Cf. A000203, A068762.

Sequence in context: A152312 A154765 A163997 * A186267 A067660 A235472

Adjacent sequences:  A067928 A067929 A067930 * A067932 A067933 A067934

KEYWORD

nonn

AUTHOR

Joseph L. Pe, Feb 22 2002

EXTENSIONS

Edited and extended by Klaus Brockhaus, Feb 28 2002

a(19)-a(24) from Donovan Johnson, Jul 26 2011

STATUS

approved

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Last modified November 18 12:16 EST 2019. Contains 329261 sequences. (Running on oeis4.)