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Smallest prime of the form (2*n)*3^m + 1 for some m >= 0, or -1 if no such prime exists.
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%I #19 Aug 30 2017 21:48:44

%S 3,5,7,73,11,13,43,17,19,61,23,73,79,29,31,97,103,37,3079,41,43,397,

%T 47,433,151,53,163,1102249,59,61,5023,193,67,613,71,73,223,229,79,241,

%U 83,757,6967,89,271,277,283,97,883,101,103,313,107,109,331,113,3079

%N Smallest prime of the form (2*n)*3^m + 1 for some m >= 0, or -1 if no such prime exists.

%C There exist even integers 2*n such that (2*n)*3^m + 1 is always composite.

%C It is conjectured that the smallest one is 125050976086 = A123159(3), therefore a(62525488043) = -1.

%C For the corresponding numbers m see A291437.

%H Robert G. Wilson v, <a href="/A291438/b291438.txt">Table of n, a(n) for n = 1..1000</a>

%e a(4) = 73 because 8*3^2 + 1 = 73 is the smallest prime of this form, since 8*3^0 + 1 = 9 and 8*3^1 + 1 = 25 are not prime.

%p a:=[]:

%p for n from 1 to 10^3 do

%p t:=-1:

%p for m from 0 to 10^3 do # this max value of m is sufficient up to n=10^3

%p if isprime((2*n)*3^m+1) then t:=m: break: fi:

%p od:

%p a:=[op(a),(2*n)*3^t+1]:

%p od:

%p a;

%t Table[If[# < 0, #, 1 + 2 n*3^#] &@ SelectFirst[Range[0, 10^3], PrimeQ[2 n*3^# + 1] &] /. k_ /; MissingQ@ k -> -1, {n, 60}] (* _Michael De Vlieger_, Aug 23 2017 *)

%o (PARI) a(n) = {my(m = 0); while (!isprime(p=(2*n)*3^m + 1), m++); p;} \\ _Michel Marcus_, Aug 25 2017

%Y Cf. A057025, A123159, A291437.

%K nonn

%O 1,1

%A _Martin Renner_, Aug 23 2017