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A124401 Indices where 2 occurs in A124151. 4
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; text; internal format)
OFFSET

1,1

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, i.e., where 1+n is 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. - R. J. Mathar, Feb 03 2010

LINKS

Table of n, a(n) for n=1..35.

FORMULA

A127936 \ A006093. - R. J. Mathar, Feb 03 2010

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 (* Robert G. Wilson v, Dec 17 2006 *)

CROSSREFS

Cf. A006093, A124205-A124209, A124164, A124178, A124181, A124185-A124187, A124189, A124200, A124154, A124163.

Sequence in context: A034784 A190280 A261786 * A292526 A151747 A088597

Adjacent sequences:  A124398 A124399 A124400 * A124402 A124403 A124404

KEYWORD

nonn

AUTHOR

Artur Jasinski, Dec 14 2006

EXTENSIONS

More terms from Robert G. Wilson v, Dec 17 2006

a(24)-a(35) from R. J. Mathar, Feb 03 2010

STATUS

approved

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Last modified February 24 18:24 EST 2018. Contains 299628 sequences. (Running on oeis4.)