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%I #11 May 26 2014 04:35:46
%S 2,2,2,3,2,2,0,2,2,3,2,3,0,2,2,7,0,2,0,2,2,3,2,3,0,2,0,7,2,2,0,3,2,3,
%T 2,2,0,5,2,7,2,3,0,2,0,3,0,2,0,2,2,3,2,2,0,2,0,19,0,3,0,5,2,3,2,3,0,2,
%U 2,3,0,13,0,2,2,3,0,2,0,3,2,19,2,5,0,2,0,7,2,2,0,3,0,3,2,2,0,2,2,7,0,3,0,3
%N Smallest prime p such that np +1 is a prime, or 0 if no such prime exists.
%C a(2n+1) = 0 if 4n+3 is composite. Conjecture: There are no other zeros.
%H Robert Israel, <a href="/A089367/b089367.txt">Table of n, a(n) for n = 1..20000</a>
%p for n from 1 to 245 do if(n mod 2 =1 and not isprime(2*n+1)) then a[n]:=0: else i:=1:while(not isprime(n*ithprime(i)+1)) do i:=i+1:od: a[n]:=ithprime(i):fi:od:seq(a[j],j=1..245); # _Sascha Kurz_, May 09 2004
%p N:= 1000: # to get a(n) for n <= 2*N
%p count:= 0:
%p a:= Vector(2*N):
%p for i from 1 to 2*N do if isprime(2*i+1) then
%p a[i]:= 2;
%p if type(i,even) then count:= count+1 fi
%p fi od:
%p for p in select(isprime,[$3..N]) while count < N do
%p for q in select(isprime,[seq(2*p*i+1,i=1..N)]) do
%p n:= (q-1)/p;
%p if a[n] = 0 then a[n]:= p;
%p count:= count+1;
%p fi
%p od
%p od:
%p A089367:= [seq(a[i],i=1..2*N)]; # _Robert Israel_, May 26 2014
%K nonn
%O 1,1
%A _Amarnath Murthy_, Nov 08 2003
%E More terms from _Sascha Kurz_, May 09 2004
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