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A210651 a(n)=least odd number k such that 3*k*2^n-1 is a prime number.

%I A210651 #18 Jul 22 2025 22:30:55
%S A210651 1,1,1,1,5,1,1,9,15,5,1,29,3,5,3,15,5,1,17,19,3,11,23,13,19,19,7,9,15,
%T A210651 71,5,19,49,1,11,15,7,1,21,39,5,11,1,19,55,11,71,39,23,29,7,61,49,15,
%U A210651 1,11,17,37,15,31,23,19,3,1,33,21,37,21,17,9,41,43,5,7,25,1,19,37,23,5,27,5,37,31,15,85,67,43,79,77
%N A210651 a(n)=least odd number k such that 3*k*2^n-1 is a prime number.
%H A210651 Pierre CAMI, <a href="/A210651/b210651.txt">Table of n, a(n) for n = 1..7725</a>
%p A210651 A210651 := proc(n)
%p A210651         for k from 1 by 2 do
%p A210651                 if isprime(3*k*2^n-1) then
%p A210651                         return k;
%p A210651                 end if;
%p A210651         end do:
%p A210651 end proc: # _R. J. Mathar_, Mar 29 2012
%o A210651 PFGW64 and SCRIPTIFY from Primeform group
%o A210651 Command: pfgw64 -f in.txt
%o A210651 in.txt file :
%o A210651 SCRIPT
%o A210651 DIM nn,0
%o A210651 DIM kk
%o A210651 DIMS tt
%o A210651 OPENFILEOUT myfile,a(n).txt
%o A210651 LABEL loopn
%o A210651 SET nn,nn+1
%o A210651 IF nn>7725 THEN END
%o A210651 SET kk,-1
%o A210651 LABEL loopk
%o A210651 SET kk,kk+2
%o A210651 SETS tt,%d,%d\,;nn;kk
%o A210651 PRP 3*kk*2^nn-1,tt
%o A210651 IF ISPRP THEN GOTO a
%o A210651 IF ISPRIME THEN GOTO a
%o A210651 GOTO loopk
%o A210651 LABEL a
%o A210651 WRITE myfile,tt
%o A210651 GOTO loopn
%Y A210651 Cf. A126717.
%K A210651 nonn
%O A210651 1,5
%A A210651 _Pierre CAMI_, Mar 27 2012