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A125567
Numbers k such that Sum_{x=2..k} (x-1)*3^(x-2) = ((2*k-3)*3^(k-1)+1)/4 is prime.
0
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
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
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