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A125570
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Numbers n such that Sum_(x=1..n) (x-1)*6^(x-1)/6 is prime.
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0
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3, 7, 8, 19, 69, 77, 104, 107, 162, 163, 399, 4787, 4818
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OFFSET
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1,1
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COMMENTS
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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
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LINKS
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MATHEMATICA
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s = 0; Do[s += (x-1)*6^(x-1)/6; If[PrimeQ[s], Print[x]], {x, 10^4}]
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CROSSREFS
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KEYWORD
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hard,more,nonn
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AUTHOR
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STATUS
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approved
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