OFFSET
1,3
COMMENTS
Original name: Numbers n such that (7*10^(2n+1)-45*10^n-7)/9 is prime.
a(12) > 10^5. - Robert Price, Nov 02 2015
REFERENCES
C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
LINKS
Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)
Makoto Kamada, Prime numbers of the form 77...77277...77
FORMULA
a(n) = (A077777(n-1)-1)/2 for n>1.
MATHEMATICA
Do[If[PrimeQ[(7*10^(2n + 1) - 45*10^n - 7)/9], Print[n]], {n, 3000}]
PROG
(PARI) is(n)=ispseudoprime((7*10^(2*n+1)-45*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
KEYWORD
nonn,base
AUTHOR
Ray Chandler, Dec 28 2010
EXTENSIONS
Name edited and a(1) = 0 inserted by M. F. Hasler, Feb 07 2020
STATUS
approved