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A077788 Numbers k such that 7*(10^k - 1)/9 - 10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime). 1
9, 11, 17, 23, 2489, 3371, 4019, 29315, 30237 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

Prime versus probable prime status and proofs are given in the author's table.

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..9.

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

Makoto Kamada, Prime numbers of the form 77...77677...77

Index entries for primes involving repunits.

FORMULA

a(n) = 2*A183181(n) + 1.

EXAMPLE

a(n)=11 -> 7*(10^11 - 1)/9 - 10^5 = 77777677777.

MATHEMATICA

Do[ If[ PrimeQ[(7*10^n - 9*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 30300, 2}] (* Robert G. Wilson v, Dec 16 2005 *)

CROSSREFS

Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.

Sequence in context: A228951 A108113 A111391 * A137018 A182391 A263722

Adjacent sequences:  A077785 A077786 A077787 * A077789 A077790 A077791

KEYWORD

more,nonn,base

AUTHOR

Patrick De Geest, Nov 16 2002

EXTENSIONS

Name corrected by Jon E. Schoenfield, Oct 31 2018

STATUS

approved

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Last modified January 23 10:50 EST 2020. Contains 331171 sequences. (Running on oeis4.)