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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
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.
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
See A332113 for the (prime and composite) near-repunit palindromes 1..131..1.
Sequence in context: A184314 A300938 A183258 * A176112 A215133 A146318
KEYWORD
nonn,base,more
AUTHOR
Patrick De Geest, Nov 16 2002
EXTENSIONS
Name corrected by Jon E. Schoenfield, Oct 31 2018
STATUS
approved