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A077788
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Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form 7*(10^a(n)-1)/9-10^[ a(n)/2 ].
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1
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OFFSET
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1,1
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COMMENTS
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Prime versus probable prime status and proofs are given in the author's table.
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REFERENCES
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C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
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LINKS
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Table of n, a(n) for n=1..9.
Patrick De Geest, World!Of Numbers, Palindromic Wing Primes (PWP's)
Makoto Kamada, Prime numbers of the form 77...77677...77
Index entries for primes involving repunits.
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FORMULA
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a(n) = 2*A183181(n)+1.
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EXAMPLE
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a(n)=11 -> 7*(10^11-1)/9-10^5 = 77777677777.
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MATHEMATICA
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Do[ If[ PrimeQ[(7*10^n - 9*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 30300, 2}] (* Robert G. Wilson v, Dec 16 2005 *)
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CROSSREFS
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Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
Sequence in context: A228951 A108113 A111391 * A137018 A182391 A263722
Adjacent sequences: A077785 A077786 A077787 * A077789 A077790 A077791
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KEYWORD
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more,nonn,base
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AUTHOR
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Patrick De Geest, Nov 16 2002
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STATUS
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approved
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