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A183182
Numbers k such that (7*10^(2*k+1) + 9*10^k - 7)/9 is prime.
2
1, 3, 39, 54, 168, 240, 5328, 6159, 24675, 52227, 113887
OFFSET
1,2
REFERENCES
C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
FORMULA
a(n) = (A077793(n) - 1)/2.
MATHEMATICA
Do[If[PrimeQ[(7*10^(2n + 1) + 9*10^n - 7)/9], Print[n]], {n, 3000}]
PROG
(PARI) is(n)=ispseudoprime((7*10^(2*n+1)+9*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,base
AUTHOR
Ray Chandler, Dec 28 2010
EXTENSIONS
a(9) from Robert Price, Oct 07 2023
a(10) from Robert Price, Oct 30 2023
a(11) from Robert Price, Aug 03 2024
STATUS
approved