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A124401
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Indices where 2 occurs in A124151.
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4
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3, 5, 8, 9, 11, 15, 21, 39, 50, 63, 83, 95, 99, 173, 350, 854, 1308, 1769, 2903, 5250, 5345, 5639, 6195, 7239, 21368, 41669, 47684, 58619, 63515, 69468, 70539, 133508, 134993, 187160, 493095
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Does 2 occur infinitely often in A124151?
The sum in A124151 is 1+n if k=1, and 1+k*(k^(2n)-1)/(k^2-1) if k>1. The indices of A124151(n)=2 are where k=1 is avoided, but where k=2 leads to a prime, ie., where 1+n s not prime but 1+2*(4^n-1)-1)/3 = (2^(2n+1)+1)/3 is prime. Therefore this sequence here is constructed by taking all n=(A000978(i)-1)/2 (the members of A127936), and eliminating cases with 1+n in A000040. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 03 2010]
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FORMULA
| A127936 \ A006093. [From R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 03 2010]
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MATHEMATICA
| f[n_] := Block[{k = 1}, While[ !PrimeQ[ Sum[k^(2j - 1), {j, n}] + 1] && k < 3, k++ ]; k]; lst = {}; Do[ If[f@n == 2, Print[n]; AppendTo[lst, n]], {n, 9250}]; lst - from Robert G. Wilson v (rgwv(at)rgwv.com), Dec 17 2006
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CROSSREFS
| Cf. A006093, A124205-A124209, A124164, A124178, A124181, A124185-A124187, A124189, A124200, A124154, A124163.
Sequence in context: A026223 A034784 A190280 * A151747 A088597 A080640
Adjacent sequences: A124398 A124399 A124400 * A124402 A124403 A124404
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KEYWORD
| nonn
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AUTHOR
| Artur Jasinski (grafix(AT)csl.pl), Dec 14 2006
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EXTENSIONS
| More terms from Robert G. Wilson v (rgwv(at)rgwv.com), Dec 17 2006
Extended beyond a(24) by R. J. Mathar (mathar(AT)strw.leidenuniv.nl), Feb 03 2010
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