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 A089367 Smallest prime p such that np +1 is a prime, or 0 if no such prime exists. 1
 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, 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, 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(2n+1) = 0 if 4n+3 is composite. Conjecture: There are no other zeros. LINKS Robert Israel, Table of n, a(n) for n = 1..20000 MAPLE 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 N:= 1000: # to get a(n) for n <= 2*N count:= 0: a:= Vector(2*N): for i from 1 to 2*N do if isprime(2*i+1) then   a[i]:= 2;   if type(i, even) then count:= count+1 fi fi od: for p in select(isprime, [\$3..N]) while count < N do   for q in select(isprime, [seq(2*p*i+1, i=1..N)]) do      n:= (q-1)/p;      if a[n] = 0 then a[n]:= p;         count:= count+1;      fi    od od: A089367:= [seq(a[i], i=1..2*N)]; # Robert Israel, May 26 2014 CROSSREFS Sequence in context: A071137 A333253 A193990 * A130192 A175064 A104564 Adjacent sequences:  A089364 A089365 A089366 * A089368 A089369 A089370 KEYWORD nonn AUTHOR Amarnath Murthy, Nov 08 2003 EXTENSIONS More terms from Sascha Kurz, May 09 2004 STATUS approved

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Last modified May 6 19:52 EDT 2021. Contains 343586 sequences. (Running on oeis4.)