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A067931
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Numbers n such that n divides the alternating sum sigma(1)-sigma(2)+sigma(3)-sigma(4)+...+((-1)^(n+1))sigma(n).
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2
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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; internal format)
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OFFSET
| 1,2
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COMMENTS
| No further term below 10^7.
a(25) > 5*10^10. - Donovan Johnson, Jul 26 2011
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EXAMPLE
| sigma(1)-sigma(2) = -2, which is divisible by 2, so 2 is a term of the sequence.
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MATHEMATICA
| s = 0; Do[s = s + (-1)^(i + 1) * DivisorSigma[1, i]; If[Mod[s, i] == 0, Print[i]], {i, 1, 10^5}]
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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, ", ")))}
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CROSSREFS
| Cf. A000203, A068762.
Sequence in context: A152312 A154765 A163997 * A186267 A067660 A103200
Adjacent sequences: A067928 A067929 A067930 * A067932 A067933 A067934
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KEYWORD
| nonn
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AUTHOR
| Joseph L. Pe (joseph_l_pe(AT)hotmail.com), Feb 22 2002
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EXTENSIONS
| Edited and extended by Klaus Brockhaus (klaus-brockhaus(AT)t-online.de), Feb 28 2002
a(19)-a(24) from Donovan Johnson (donovan.johnson(AT)yahoo.com), Jul 26 2011
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