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

1,1

Links

Table of n, a(n) for n=1..58.

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

Example

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

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]

Prog

(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

Crossrefs

Cf. A008908.

Sequence in context: A049582 A220273 A276698 * A031377 A019357 A024920

Adjacent sequences: A260798 A260799 A260800 * A260802 A260803 A260804

Keyword

easy,nonn

Author

Ivan N. Ianakiev, Jul 31 2015

Status

approved