A077779 - OEIS

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A077779

Numbers k such that (10^k - 1)/9 + 2*10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime).

3

3, 5, 39, 195, 19637

(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.

a(6) > 2*10^5. - Robert Price, Apr 02 2016

The number k = 1 would also correspond to a prime, 3, but not "near-repdigit" or "wing" in a strict sense. - M. F. Hasler, Feb 09 2020

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..5.

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

Makoto Kamada, Prime numbers of the form 11...11311...11

Index entries for primes involving repunits.

FORMULA

a(n) = 2*A107123(n+1) + 1.

EXAMPLE

5 is a term because (10^5 - 1)/9 + 2*10^2 = 11311.

MATHEMATICA

Do[ If[ PrimeQ[(10^n + 18*10^Floor[n/2] - 1)/9], Print[n]], {n, 3, 20000, 2}] (* Robert G. Wilson v, Dec 16 2005 *)

CROSSREFS

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

See A332113 for the (prime and composite) near-repunit palindromes 1..131..1.

Sequence in context: A184314 A300938 A183258 * A176112 A215133 A146318

Adjacent sequences: A077776 A077777 A077778 * A077780 A077781 A077782

KEYWORD

nonn,base,more

AUTHOR

Patrick De Geest, Nov 16 2002

EXTENSIONS

Name corrected by Jon E. Schoenfield, Oct 31 2018

STATUS

approved