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A154022
a(n) = 5*A097780(n-2).
2
0, 5, 125, 3120, 77875, 1943755, 48516000, 1210956245, 30225390125, 754423796880, 18830369531875, 470004814499995, 11731289992968000, 292812245009700005, 7308574835249532125, 182421558636228603120
OFFSET
1,2
COMMENTS
If a(n)=x and a(n+1)=y, then (x^2+y^2)/(xy+1)=25.
FORMULA
a(n) = +25*a(n-1) -a(n-2).
G.f.: 5*x^2/(1 -25*x +x^2). - R. J. Mathar, Jan 05 2011
MATHEMATICA
CoefficientList[Series[(5*z)/(z^2-25*z+1), {z, 0, 25}], z] (* Vincenzo Librandi, Jan 29 2012 *)
LinearRecurrence[{25, -1}, {0, 5}, 20] (* Harvey P. Dale, Mar 06 2018 *)
PROG
(PARI) concat(0, Vec(5/(1-25*x+x^2)+O(x^98))) \\ Charles R Greathouse IV, Dec 27 2011
(Magma) I:=[0, 5]; [n le 2 select I[n] else 25*Self(n-1)-Self(n-2): n in [1..20]]; // Vincenzo Librandi, Jan 29 2012
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Jan 04 2009
EXTENSIONS
Edited by N. J. A. Sloane, Jun 23 2010 at the suggestion of Joerg Arndt.
STATUS
approved