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

Author

Ryan Propper, Jan 01 2007

Keywords

• hard
• more
• nonn

Comments

No more terms through 10^4.

No more terms through 5*10^4. - Michael S. Branicky, Jun 30 2024

Crossrefs

Cf. A119529.

Programs

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 *)

Extensions

Name simplified by Jon E. Schoenfield, Sep 23 2018