OFFSET
1,2
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
LINKS
Makoto Kamada, Prime numbers of the form 799...993.
Erich Friedman, What's Special About This Number?.
FORMULA
a(n) = A101155(n) + 1.
EXAMPLE
73, 7993, 799993, 7999993, etc. are primes.
MATHEMATICA
Do[ If[ PrimeQ[8*10^n - 7], Print[n]], {n, 10000}]
PROG
(PARI) is(n)=ispseudoprime(8*10^n-7) \\ Charles R Greathouse IV, Feb 20 2017
CROSSREFS
KEYWORD
more,nonn
AUTHOR
Robert G. Wilson v, Oct 01 2004
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
STATUS
approved