A099190 Numbers n such that 8*10^n-7 is prime.

1, 3, 5, 6, 10, 12, 13, 39, 48, 54, 64, 82, 147, 148, 360, 399, 1638, 1876, 2146, 2194, 15789, 23074, 38466, 68400

Offset: 1

Author

Robert G. Wilson v, Oct 01 2004

Keywords

• more
• nonn

Comments

Also numbers n such that 7*10^n + 9*R_n - 6 is prime, where R_n = 11...1 is the repunit (A002275) of length n.

Primes of the form 7*10^n+9R_n-6 are the only primes which are one more than twice their reversal.

a(25) > 2*10^5. - Robert Price, Nov 11 2015

Examples

			73, 7993, 799993, 7999993, etc. are primes.
		

Links

• Makoto Kamada, Prime numbers of the form 799...993.
• Erich Friedman, What's Special About This Number?.
• Index entries for primes involving repunits.

Crossrefs

Cf. A002275, A099181, A101155.

Programs

Mathematica

Do[ If[ PrimeQ[8*10^n - 7], Print[n]], {n, 10000}]

PARI

is(n)=ispseudoprime(8*10^n-7) \\ Charles R Greathouse IV, Feb 20 2017

Formula

a(n) = A101155(n) + 1.

Extensions

a(19) & a(20) from Robert G. Wilson v, Jan 19 2005

a(21)-a(24) from Kamada data by Robert Price, Dec 14 2010