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A119720 Numbers k such that 7^k + 2 is semiprime. 0

%I

%S 1,2,6,9,12,14,26,33,44,54,126,141,830,2436,3534,3846,4152,4460,4974,

%T 13160,13200,13832

%N Numbers k such that 7^k + 2 is semiprime.

%C Or, (7^n + 2)/3 is prime.

%e 12 is a term because 7^12 + 2 = 13841287203 = 3*4613762401 and 4613762401 is prime.

%t Select[Range[2100], PrimeQ[(7^# + 2)/3] &] (* _Stefan Steinerberger_, Jul 22 2006 *)

%K nonn,more

%O 1,2

%A _Zak Seidov_, Jun 13 2006

%E More terms from _Stefan Steinerberger_ and _Joshua Zucker_, Jun 21 2006

%E a(14) from Herman Jamke (hermanjamke(AT)fastmail.fm), Mar 30 2008

%E a(15)-a(22) from _Sean A. Irvine_, Dec 16 2009

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Last modified September 19 04:52 EDT 2019. Contains 327187 sequences. (Running on oeis4.)