A217095 Numbers n such that (10^n + 9^n)/19 is prime.

7, 67, 73, 1091, 1483, 10937

Offset: 1

Author

Robert Price, Feb 17 2013

Keywords

• hard
• nonn
• more

Comments

The numbers n themselves (7, 67, 73, ...) are also prime.

a(7) > 10^5.

Crossrefs

Cf. A057175, A125956, A128339, A181141, A187819.

Programs

Mathematica

Select[ Prime[ Range[1, 100000] ], PrimeQ[ (10^# + 9^#)/19 ]& ]

PARI

is(n)=isprime((10^n+9^n)/19) \\ Charles R Greathouse IV, Feb 17 2017