Types of Nbody ODE Integration Methods
June 9, 1996
36
Euler's method assumes that the acceleration at time
t
is a good approximation for
the acceleration for the entire interval from
t
to
t+h
. A better approximation would be the
average of the acceleration at time
t
and at time
t+h
. This can be done by using Euler's
method to predict the value of
x(t+h)
and then correcting it as follows:
This method improves Euler's method from a first order to a second order formula
at the cost of one additional evaluation of
f()
. While this is an improvement, it is still too
low of an order to be useful.
Sources: (13:70,15:207,19:4178)
The AdamMoulton's method is similar to the AdamBashford method except the
formula uses the future value
, instead of just previous values of
f()
. To get this
initial estimate of
, we use the AdamBashford formula as a predictor, and then
use the AdamMoulton formula to correct it. So the method has this form:
Where
r, s, a
1
, ... , a
n
, b
1
, ..., b
n
,
are constants
Sources: (14:590,12:623,4:3824,1:3468)
y
x
t
(
)
hf
x
t
(
)
(
)
+
=
x
t
h
+
(
)
x
t
(
)
h
f
y
(
)
f
x
t
(
)
(
)
+
2
(
)
+
=
E
O
h
3
(
)
=
t
true path
t
t+h
FIGURE 20. Graph of the Modified Euler's method
corrected location
x
x(t)
f(x(t))
f(y)
predicted location
f
x
t
h
+
(
)
(
)
f
x
t
h
+
(
)
(
)
y
x
t
(
)
hr
a
1
f
x
t
(
)
(
)
a
2
f
x
t
h

(
)
(
)

a
3
f
x
t
2
h

(
)
(
)
...

+
[
]
+
=
x
t
h
+
(
)
x
t
(
)
hs
b
1
f
y
(
)
b
2
f
x
t
(
)
(
)
b
3
f
x
t
h

(
)
(
)

...
+
+
[
]
+
=
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