A125567 Numbers k such that Sum_{x=2..k} (x-1)*3^(x-2) = ((2*k-3)*3^(k-1)+1)/4 is prime.

3, 6, 10, 135, 186, 542, 1162, 1726, 4179, 13910, 14514, 78915, 170026

Offset: 1

Author

Ryan Propper, Jan 01 2007

Keywords

• hard
• more
• nonn

Comments

a(14) > 2*10^5. - Robert Price, Jan 25 2015

Examples

			For k = 3, 6, 10 the corresponding primes are 7, 547, 83653.
		

Crossrefs

Cf. A119529.

Programs

Mathematica

s = 0; Do[s += (x-1)*3^(x-2); If[PrimeQ[s], Print[x]], {x, 10^4}]

Extensions

a(10) from Max Alekseyev, Dec 10 2011

a(11)-a(13) from Robert Price, Jan 25 2015