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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
Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)
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
Sequence in context: A064600 A174874 A109904 * A102872 A102873 A356847
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.)