A273010 Numbers n such that (9^n - 7^n)/2 is prime.

3, 5, 7, 4703, 30113, 835391

Offset: 1

Author

Tim Johannes Ohrtmann, May 13 2016

Keywords

• nonn
• more
• hard

Comments

All terms are prime.

The corresponding primes: 193, 21121, 1979713, ...

Links

• Jon Grantham and Andrew Granville, Fibonacci primes, primes of the form 2^n-k and beyond, arXiv:2307.07894 [math.NT], 2023.

Crossrefs

Cf. A081202, A173718, A128346, A059803.

Programs

Mathematica

Select[Range[1, 10000], PrimeQ[(9^# - 7^#)/2] &]

PARI

for(n=1,10000, if(isprime((9^n - 7^n)/2), print1(n,", ")))

Extensions

a(6) from Jon Grantham, Jul 29 2023