A077796 Numbers k such that 7*(10^k - 1)/9 + 2*10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime).

3, 5, 17, 39, 41, 425, 561, 1775, 2043, 11031, 16233, 23705

Offset: 1

Author

Patrick De Geest, Nov 16 2002

Keywords

• more
• nonn
• base

Comments

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

a(13) > 2*10^5. - Robert Price, Jan 19 2016

Examples

			17 is a term because 7*(10^17 - 1)/9 + 2*10^8 = 77777777977777777.
		

References

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

Links

• Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)
• Makoto Kamada, Prime numbers of the form 77...77977...77
• Index entries for primes involving repunits.

Crossrefs

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

Programs

Mathematica

Do[ If[ PrimeQ[(7*10^n + 18*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 23800, 2}] (* Robert G. Wilson v, Dec 16 2005 *)

Formula

a(n) = 2*A183183(n) + 1.

Extensions

Name corrected by Jon E. Schoenfield, Oct 31 2018