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 A077787 Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form (10^a(n)-1)/9+5*10^[ a(n)/2 ]. 1
 21, 29, 81, 119, 321, 825, 1121, 2579, 3693 (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(10) > 2*10^5. - Robert Price, Sep 28 2015 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 11...11611...11 FORMULA a(n) = 2*A107126(n)+1. EXAMPLE a(n)=21 -> (10^21-1)/9+5*10^10 = 111111111161111111111. MATHEMATICA Do[ If[ PrimeQ[(10^n + 45*10^Floor[n/2] - 1)/9], Print[n]], {n, 3, 4000, 2}] (* Robert G. Wilson v, Dec 16 2005 *) CROSSREFS Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187. Sequence in context: A004196 A211253 A209975 * A039372 A043195 A043975 Adjacent sequences:  A077784 A077785 A077786 * A077788 A077789 A077790 KEYWORD more,nonn,base AUTHOR Patrick De Geest, Nov 16 2002 STATUS approved

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