A089727 - OEIS

%I #13 Jul 19 2015 01:30:08

%S 2,5,7,17,11,37,43,41,73,101,89,109,157,197,211,257,239,307,229,401,

%T 421,463,461,577,601,677,541,757,523,811,683,929,991,1123,1051,1297,

%U 1259,1217,1483,1601,1559,1723,1721,1453,1801,1979,2069,2161,1667,2351,2551

%N Largest prime of the form n*k+1, k <= n.

%H Vincenzo Librandi, <a href="/A089727/b089727.txt">Table of n, a(n) for n = 1..1000</a>

%e a(5)=11 because 5*2+1=11 is prime, whereas 5*5+1=26, 5*4+1=21 and 5*3+1=16 are composite.

%t f[n_] := Block[{k = n}, While[ !PrimeQ[k*n + 1], k-- ]; k*n + 1]; Array[f, 51] (* _Robert G. Wilson v_, Dec 17 2009 *)

%o (PARI) A089727(n) = {local(k);k=n;while(!isprime(n*k+1),k--);n*k+1} \\ _Michael B. Porter_, Dec 17 2009

%Y Cf. A089726.

%K nonn,easy

%O 1,1

%A _Amarnath Murthy_, Nov 18 2003

%E More terms from _Hugo Pfoertner_, Apr 01 2004