A183180 - OEIS

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A183180

Numbers k such that (7*10^(2k+1) - 18*10^k - 7)/9 is prime.

0, 1, 7, 13, 58, 129, 253, 1657, 2244, 2437, 7924, 9903, 11899, 18157, 18957, 23665, 105609

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OFFSET

1,3

COMMENTS

a(17) > 10^5. - Robert Price, Jun 23 2017

0 could be considered part of this sequence since the formula evaluates to 5 which is a degenerate form of the near-repdigit palindrome 777...77577...777 with 0 occurrences of the digit 7. - Robert Price, Jun 23 2017

REFERENCES

C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.

LINKS

Table of n, a(n) for n=1..17.

Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)

Makoto Kamada, Prime numbers of the form 77...77577...77

Index entries for primes involving repunits.

FORMULA

a(n) = (A077785(n) - 1)/2.

MATHEMATICA

Do[If[PrimeQ[(7*10^(2n + 1) - 18*10^n - 7)/9], Print[n]], {n, 3000}]

PROG