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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
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.
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
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