login
A099190
Numbers n such that 8*10^n-7 is prime.
5
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
1,2
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
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