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A125569
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Numbers k such that Sum_{j=1..k-1} j*5^(j-1) is prime.
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0
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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.
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LINKS
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MATHEMATICA
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s = 0; Do[s += (x-1)*5^(x-1)/5; If[PrimeQ[s], Print[x]], {x, 10^4}]
Select[Range[2100], PrimeQ[Sum[x*5^(x-1), {x, #-1}]]&] (* Harvey P. Dale, Feb 09 2023 *)
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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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EXTENSIONS
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STATUS
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approved
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