cp's OEIS Frontend

A258868 a(n) is the smallest integer >= a(n-1) such that prime(n)*2^a(n)-1 is a prime number.

%I A258868 #16 Sep 23 2016 11:52:52
%S A258868 1,1,2,5,26,287,356,395,544,11008,21957,32125,42450,50867,55408,
%T A258868 206970,358276,384287,403461,735802,783831,969795,1192950,1383108
%N A258868 a(n) is the smallest integer >= a(n-1) such that prime(n)*2^a(n)-1 is a prime number.
%H A258868 K. Bonath, <a href="http://www.rieselprime.de">Riesel and Proth Prime Database</a> (2015)
%e A258868 2*2^1-1=3 prime so a(1)=1.
%e A258868 3*2^1-1=5 prime so a(2)=1.
%e A258868 5*2^1-1=9 composite, 5*2^2-1=19 prime so a(3)=2.
%p A258868 A258868 := proc(n)
%p A258868     option remember;
%p A258868     if n = 0 then
%p A258868         0;
%p A258868     else
%p A258868         for a from procname(n-1) do
%p A258868             ithprime(n)*2^a-1 ;
%p A258868             if isprime(%) then
%p A258868                 return a;
%p A258868             fi ;
%p A258868         end do:
%p A258868     end if;
%p A258868 end proc: # _R. J. Mathar_, Sep 23 2016
%t A258868 lst={1};Do[x=Last[lst];Label[begin];
%t A258868 If[PrimeQ[Prime[n]*2^x-1],AppendTo[lst,x],x=x+1;Goto[begin]],{n,2,9}];lst
%t A258868 (* _Ivan N. Ianakiev_, Jun 19 2015 *)
%o A258868 (PARI) first(n)=my(t,p); vector(n,i, p=prime(i); while(!ispseudoprime(p<<t-1), t++); t) \\ _Charles R Greathouse IV_, Jul 03 2015
%Y A258868 Cf. A128979.
%K A258868 nonn
%O A258868 1,3
%A A258868 _Pierre CAMI_, Jun 13 2015