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A077777
Numbers k such that 7*(10^k - 1)/9 - 5*10^floor(k/2) is a palindromic wing prime (a.k.a. near-repdigit palindromic prime).
2
3, 7, 15, 21, 25, 961, 1899, 3891, 15097, 17847
OFFSET
1,1
COMMENTS
Prime versus probable prime status and proofs are given in the author's table.
a(11) > 2*10^5. - Robert Price, Nov 02 2015
A183178(1) = 0 would correspond to an initial term 1 in this sequence which yields the prime 2 (which has a "wing" of length 0 and is a palindrome and repdigit but not near-repdigit). - M. F. Hasler, Feb 08 2020
REFERENCES
C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
FORMULA
a(n) = 2*A183178(n+1) + 1.
EXAMPLE
15 is a term because 7*(10^15 - 1)/9 - 5*10^7 = 777777727777777.
MATHEMATICA
Do[ If[ PrimeQ[(7*10^n - 45*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 1000, 2}] (* Robert G. Wilson v, Dec 16 2005 *)
PROG
(PARI) for(k=1, oo, ispseudoprime(10^k\9*7-5*10^(k\2))&&print1(k", ")) \\ M. F. Hasler, Feb 08 2020
KEYWORD
more,nonn,base
AUTHOR
Patrick De Geest, Nov 16 2002
EXTENSIONS
Name corrected by Jon E. Schoenfield, Oct 31 2018
STATUS
approved