A261578 Numbers m such that (4^m + 17) / 3 is prime.

1, 2, 5, 8, 11, 23, 26, 59, 83, 89, 116, 1103, 1568, 5768, 13376, 17810, 18614, 66209, 167933, 188318

Offset: 1

Author

Vincenzo Librandi, Aug 26 2015

Keywords

• nonn
• more

Comments

After 1, m is of the form 3*k+2. In fact, for m = 3*k or 3*k+1, 4^n+17 is divisible by 9 and 7, respectively. [Bruno Berselli, Aug 26 2015]

a(21) > 300000. - Robert Price, Apr 04 2017

Examples

			2 is in the sequence because (4^2+17)/3 = 11 is prime.
5 is in the sequence because (4^5+17)/3 = 347 is prime.
		

Crossrefs

Cf. similar sequences listed in A261539.

Programs

Magma

[n: n in [0..1000] | IsPrime((4^n+17) div 3)];

Mathematica

Select[Range[0, 5000], PrimeQ[(4^# + 17)/3] &]

PARI

for(n=1, 1e3, if(isprime((4^n+17)/3), print1(n", "))) \\ Altug Alkan, Sep 14 2015

Extensions

a(14)-a(15) from Vincenzo Librandi, Sep 14 2015

a(16)-a(20) from Robert Price, Feb 01 2017