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A077781
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Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form 7*(10^a(n)-1)/9-3*10^[ a(n)/2 ].
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1
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5, 7, 13, 47, 73, 139, 1123, 1447, 6877, 8209, 18041, 27955, 39311
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,1
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COMMENTS
| Prime versus probable prime status and proofs are given in the author's table.
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REFERENCES
| C. Caldwell and H. Dubner, "Journal of Recreational Mathematics", Volume 28, No. 1, 1996-97, pp. 1-9.
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LINKS
| 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.
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FORMULA
| a(n) = 2*A183179(n)+1.
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EXAMPLE
| a(n)=7 -> 7*(10^7-1)/9-3*10^3 = 7774777.
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MATHEMATICA
| Do[ If[ PrimeQ[(7*10^n - 27*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 39400, 2}] (from Robert G. Wilson v (rgwv(at)rgwv.com), Dec 16 2005)
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CROSSREFS
| Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
Sequence in context: A064600 A174874 A109904 * A102872 A102873 A158892
Adjacent sequences: A077778 A077779 A077780 * A077782 A077783 A077784
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KEYWORD
| more,nonn,base
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AUTHOR
| Patrick De Geest (pdg(AT)worldofnumbers.com), Nov 16 2002.
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