A125570 Numbers n such that Sum_(x=1..n) (x-1)*6^(x-1)/6 is prime.

3, 7, 8, 19, 69, 77, 104, 107, 162, 163, 399, 4787, 4818

Offset: 1

Author

Ryan Propper, Jan 01 2007

Keywords

• hard
• more
• nonn

Comments

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

Crossrefs

Cf. A119529.

Programs

Mathematica

s = 0; Do[s += (x-1)*6^(x-1)/6; If[PrimeQ[s], Print[x]], {x, 10^4}]