A164620 Primes p such that 1 +p*floor(p/2) is also prime.

2, 5, 13, 17, 41, 61, 89, 97, 101, 113, 149, 173, 229, 241, 281, 317, 349, 353, 373, 397, 409, 421, 433, 461, 509, 521, 661, 673, 761, 853, 881, 937, 941, 1013, 1093, 1109, 1249, 1289, 1297, 1373, 1433, 1549, 1741, 1753, 1913, 2113, 2213, 2269, 2281, 2297

Offset: 1

Author

Vladimir Joseph Stephan Orlovsky, Aug 17 2009

Keywords

• nonn
• easy

Examples

			p=2 qualifies since 2*1+1=3 is prime. p=5 qualifies since 5*2+1=11 is prime.
		

Links

• Vincenzo Librandi, Table of n, a(n) for n = 1..1000

Crossrefs

Cf. A008846, A020882, A158708.

Programs

Mathematica

lst={};Do[p=Prime[n];If[PrimeQ[p*Floor[p/2]+1],AppendTo[lst,p]],{n,3*6!}]; lst
Select[Prime[Range[350]],PrimeQ[1+#*Floor[#/2]]&] (* Harvey P. Dale, Apr 07 2015 *)

Extensions

Comments turned into examples by R. J. Mathar, Sep 17 2009