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A077785
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Palindromic wing primes (a.k.a. near-repdigit palindromes) of the form 7*(10^a(n)-1)/9-2*10^[ a(n)/2 ].
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1
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3, 15, 27, 117, 259, 507, 3315, 4489, 4875, 15849, 19807, 23799, 36315, 37915
(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...77577...77
Index entries for primes involving repunits.
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FORMULA
| a(n) = 2*A183180(n)+1.
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EXAMPLE
| a(n)=15 -> 7*(10^15-1)/9-2*10^7 = 777777757777777.
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MATHEMATICA
| Do[ If[ PrimeQ[(7*10^n - 18*10^Floor[n/2] - 7)/9], Print[n]], {n, 3, 40000, 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: A147041 A002259 A050848 * A015646 A067144 A018784
Adjacent sequences: A077782 A077783 A077784 * A077786 A077787 A077788
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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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