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%I #15 Sep 23 2018 22:05:14
%S 3,6,10,135,186,542,1162,1726,4179,13910,14514,78915,170026
%N Numbers k such that Sum_{x=2..k} (x-1)*3^(x-2) = ((2*k-3)*3^(k-1)+1)/4 is prime.
%C a(14) > 2*10^5. - _Robert Price_, Jan 25 2015
%e For k = 3, 6, 10 the corresponding primes are 7, 547, 83653.
%t s = 0; Do[s += (x-1)*3^(x-2); If[PrimeQ[s], Print[x]], {x, 10^4}]
%Y Cf. A119529.
%K hard,more,nonn
%O 1,1
%A _Ryan Propper_, Jan 01 2007
%E a(10) from _Max Alekseyev_, Dec 10 2011
%E a(11)-a(13) from _Robert Price_, Jan 25 2015
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