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A107125
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Numbers n such that (10^(2n+1)+36*10^n-1)/9 is prime.
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1
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0, 1, 7, 45, 115, 681, 1248, 2481, 2689, 6198, 13197
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| n is in the sequence iff the palindromic number 1(n).5.1(n) is prime (dot between numbers means concatenation). If n is in the sequence then n is not of the forms 3m+2, 18m+12, 18m+14, 22m+4, 22m+6, etc. (the proof is easy).
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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 11...11511...11
Index entries for primes involving repunits.
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FORMULA
| a(n) = (A077783(n)-1)/2.
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EXAMPLE
| 1248 is in the sequence because (10^(2*1248+1)+36*10^1248-1)/9=1(1248).5.1(1248) is prime.
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MATHEMATICA
| Do[If[PrimeQ[(10^(2n + 1) + 36*10^n - 1)/9], Print[n]], {n, 2200}]
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CROSSREFS
| Cf. A004023, A077775-A077798, A107123-A107127, A107648, A107649, A115073, A183174-A183187.
Sequence in context: A156374 A171493 A153492 * A206808 A197369 A059937
Adjacent sequences: A107122 A107123 A107124 * A107126 A107127 A107128
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KEYWORD
| nonn
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AUTHOR
| Farideh Firoozbakht (mymontain(AT)yahoo.com), May 19 2005
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EXTENSIONS
| Edited by Ray Chandler (rayjchandler(AT)sbcglobal.net), Dec 28 2010
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