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

1,1

Comments

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

Links

Table of n, a(n) for n=1..13.

Example

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

Mathematica

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

Crossrefs

Cf. A119529.

Sequence in context: A308849 A354000 A368173 * A254957 A124266 A137941

Adjacent sequences: A125564 A125565 A125566 * A125568 A125569 A125570

Keyword

hard,more,nonn

Author

Ryan Propper, Jan 01 2007

Extensions

a(10) from Max Alekseyev, Dec 10 2011

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

Status

approved