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 A077781 Numbers k such that 7*(10^k - 1)/9 - 3*10^floor(k/2) is a palindromic wing prime (also known as near-repdigit palindromic prime). 2
 5, 7, 13, 47, 73, 139, 1123, 1447, 6877, 8209, 18041, 27955, 39311, 64801 (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(15) > 2*10^5. - Robert Price, Nov 23 2015 REFERENCES C. Caldwell and H. Dubner, The near repdigit primes A(n-k-1)B(1)A(k), especially 9(n-k-1)8(1)9(k), Journal of Recreational Mathematics, Volume 28, No. 1, 1996-97, pp. 1-9. LINKS Table of n, a(n) for n=1..14. Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's) Makoto Kamada, Prime numbers of the form 77...77477...77 Index entries for primes involving repunits. FORMULA a(n) = 2*A183179(n) + 1. EXAMPLE 7 is a term because 7*(10^7 - 1)/9 - 3*10^3 = 7774777. MATHEMATICA Do[ If[ PrimeQ[(7*10^n - 27*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 39400, 2}] (* Robert G. Wilson v, Dec 16 2005 *) PROG (PARI) for(n=1, 1e3, if(ispseudoprime((7*10^(2*n+1)-27*10^n-7)/9), print1(2*n+1, ", "))) \\ Altug Alkan, Nov 23 2015 CROSSREFS Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187. Sequence in context: A064600 A174874 A109904 * A102872 A102873 A356847 Adjacent sequences: A077778 A077779 A077780 * A077782 A077783 A077784 KEYWORD more,nonn,base AUTHOR Patrick De Geest, Nov 16 2002 EXTENSIONS a(14) from Robert Price, Nov 23 2015 Name corrected by Jon E. Schoenfield, Oct 31 2018 STATUS approved

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Last modified April 22 14:18 EDT 2024. Contains 371903 sequences. (Running on oeis4.)