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A125569
Numbers k such that Sum_{j=1..k-1} j*5^(j-1) is prime.
1
3, 10, 11, 38, 735, 1083, 2063
OFFSET
1,1
COMMENTS
No more terms through 10^4.
No more terms through 5*10^4. - Michael S. Branicky, Jun 30 2024
MATHEMATICA
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 *)
CROSSREFS
Cf. A119529.
Sequence in context: A119080 A119221 A119219 * A201156 A309525 A253807
KEYWORD
hard,more,nonn
AUTHOR
Ryan Propper, Jan 01 2007
EXTENSIONS
Name simplified by Jon E. Schoenfield, Sep 23 2018
STATUS
approved