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A183183
Numbers k such that (7*10^(2*k+1)+18*10^k-7)/9 is prime.
2
1, 2, 8, 19, 20, 212, 280, 887, 1021, 5515, 8116, 11852
OFFSET
1,2
COMMENTS
a(13) > 10^5. - Robert Price, Jan 19 2016
REFERENCES
C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
FORMULA
a(n) = (A077796(n)-1)/2.
MATHEMATICA
Do[If[PrimeQ[(7*10^(2n + 1) + 18*10^n - 7)/9], Print[n]], {n, 3000}]
PROG
(PARI) is(n)=ispseudoprime((7*10^(2*n+1)+18*10^n-7)/9) \\ Charles R Greathouse IV, Jun 13 2017
KEYWORD
nonn,more,base
AUTHOR
Ray Chandler, Dec 28 2010
STATUS
approved