A125569 Numbers k such that Sum_{j=1..k-1} j*5^(j-1) is prime.

3, 10, 11, 38, 735, 1083, 2063

Offset

1,1

Comments

No more terms through 10^4.

Links

Table of n, a(n) for n=1..7.

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

Adjacent sequences: A125566 A125567 A125568 * A125570 A125571 A125572

Keyword

hard,more,nonn

Author

Ryan Propper, Jan 01 2007

Extensions

Name simplified by Jon E. Schoenfield, Sep 23 2018

Status

approved