A260801 Primes p such that A008908(p) is also prime.

2, 7, 17, 29, 31, 71, 89, 107, 113, 127, 131, 157, 181, 223, 239, 263, 271, 277, 281, 283, 313, 337, 379, 409, 419, 421, 431, 503, 547, 571, 577, 691, 701, 727, 757, 809, 821, 857, 883, 947, 953, 971, 1031, 1109, 1129, 1153, 1163, 1231, 1283, 1291, 1327, 1361, 1447, 1487, 1531, 1559, 1567, 1583

Offset: 1

Author

Ivan N. Ianakiev, Jul 31 2015

Keywords

• easy
• nonn

Examples

			7 is prime and A008908(7) is 17, which is also prime. Therefore, 7 is in the sequence.
		

Links

• Index entries for sequences related to 3x+1 (or Collatz) problem

Crossrefs

Cf. A008908.

Programs

Mathematica

collatz[n_]:=If[EvenQ[n],n/2,3*n+1];
seq[n_]:=Length[NestWhileList[collatz,n,#!= 1&]]
Select[Flatten[Position[seq/@Range[7!],_?PrimeQ]],PrimeQ]

PARI

T(n)=c=0;while(n!=1,if(n%2,n=3*n+1;c++);if(!(n%2),n=n/2;c++));c+1
forprime(p=1,10^3,if(isprime(T(p)),print1(p,", "))) \\ Derek Orr, Aug 27 2015