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0 Introduction
In the past over two decades, symplectic methods for Hamiltonian systems have formed
a very important category in the numerical solution of ordinary differential equations. For
differential equations with particular structures, it has become a common practice to require
numerical algorithms to adapt the special structure of the problems and preserve as much as
possible the qualitative behavior of the true solutions. Recent survey of structure-preserving
algorithms for ordinary differential equations, we refer to [1, 2].
Oscillatory systems often arise in different fields of applied sciences such as the Kepler
problem, the outer solar system, the H´enon-Heiles model and problems in molecular dynamics,
etc (see [1, 3] for example). Several of these problems can be expressed in the following system
of second order differential equations:
y
00
+ ω
2
y(t) = f(y(t)), y(t
0
) = y
0
, y
0
(t
0
) = y
0
0
, (1)
where ω is the main frequency, y : R → R
d
, and f(y) is the perturbing force with the form
f(y) = −∇U(y). The system (1) is in fact a Hamiltonian system with Hamiltonian H(y
0
, y) =
1
2
y
0T
y
0
+
1
2
ω
2
y
T
y + U(y).
In order to deal with the problem (1), some novel approaches to exploring numerical
methods have been proposed and we refer to [4, 5, 6, 7, 8, 9, 10, 11, 12] for example. For problem
(1), Tocino et al. propose a scheme of modified RKN methods and discuss the symplecticity
conditions in [7]. In this paper, we apply trigonometrical fitting technique to the modified
RKN methods to achieve the so called trigonometrically fitted RKN methods for problem
(1). The corresponding symplecticity conditions for trigonometrically fitted RKN methods
are obtained by using the results given in [7]. Based on the symplecticity conditions and
composition methods, we propose some practical explicit symplectic trigonometrically fitted
RKN methods. Three numerical experiments are carried out to demonstrate the efficiency of
these new methods.
1 Trigonometrically fitted RKN methods
For the numerical solution of problem (1), modified RKN methods of the following form
are introduced in [7]:
Y
i
= C
i
y
n
+ hD
i
y
0
n
+ h
2
s
P
j=1
a
ij
f(Y
j
), i = 1, 2, · · · , s,
y
n+1
= Cy
n
+ hDy
0
n
+ h
2
s
P
i=1
β
i
f(Y
i
),
y
0
n+1
= Ay
0
n
+
B
h
y
n
+ h
s
P
i=1
b
i
f(Y
i
),
(2)
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