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A125570 Numbers n such that Sum_(x=1..n) (x-1)*6^(x-1))/6 is prime. 0
3, 7, 8, 19, 69, 77, 104, 107, 162, 163, 399, 4787, 4818 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

No more terms through 10^4.

Note that Sum(x=1,n,(x-1)*6^(x-1))/6 = (6^(n-1)*(5*n-6)+1)/25. Therefore this sequence consists of n such that (6^(n-1)*(5*n-6)+1)/25 is prime. - Max Alekseyev, Oct 18 2008

LINKS

Table of n, a(n) for n=1..13.

MATHEMATICA

s = 0; Do[s += (x-1)*6^(x-1)/6; If[PrimeQ[s], Print[x]], {x, 10^4}]

CROSSREFS

Cf. A119529.

Sequence in context: A122227 A259571 A291212 * A268111 A244532 A037208

Adjacent sequences:  A125567 A125568 A125569 * A125571 A125572 A125573

KEYWORD

hard,more,nonn

AUTHOR

Ryan Propper, Jan 01 2007

STATUS

approved

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Last modified January 17 20:36 EST 2020. Contains 330987 sequences. (Running on oeis4.)