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A002320
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a(n) = 5*a(n-1) - a(n-2).
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6
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1, 3, 14, 67, 321, 1538, 7369, 35307, 169166, 810523, 3883449, 18606722, 89150161, 427144083, 2046570254, 9805707187, 46981965681, 225104121218, 1078538640409, 5167589080827, 24759406763726, 118629444737803
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OFFSET
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0,2
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COMMENTS
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Together with A002310 these are the two sequences satisfying the requirement that (a(n)^2 + a(n-1)^2)/(1 - a(n)*a(n-1)) be an integer; in both cases this integer is -5. - Floor van Lamoen, Oct 26 2001
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REFERENCES
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From a posting to Netnews group sci.math by ksbrown(AT)seanet.com (K. S. Brown) on Aug 15 1996.
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LINKS
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FORMULA
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a(n) = Sum_{k = 0..n} A238731(n,k)*2^k. - _Philippe Deléham, Mar 05 2014
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MATHEMATICA
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LinearRecurrence[{5, -1}, {1, 3}, 30] (* Harvey P. Dale, Nov 13 2014 *)
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PROG
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(Haskell)
a002320 n = a002320_list !! n
a002320_list = 1 : 3 :
(zipWith (-) (map (* 5) (tail a002320_list)) a002320_list)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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Joe Keane (jgk(AT)jgk.org)
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STATUS
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approved
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