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The function drawdf
draws the direction field of a first-order
Ordinary Differential Equation (ODE) or a system of two autonomous
first-order ODE’s.
Since this is an additional package, in order to use it you must first
load it with load("drawdf")
. Drawdf is built upon the draw
package, which requires Gnuplot 4.2.
To plot the direction field of a single ODE, the ODE must be written in the form:
dy -- = F(x,y) dx
and the function F should be given as the argument for
drawdf
. If the independent and dependent variables are not x,
and y, as in the equation above, then those two variables should
be named explicitly in a list given as an argument to the drawdf command
(see the examples).
To plot the direction field of a set of two autonomous ODE’s, they must be written in the form
dx dy -- = G(x,y) -- = F(x,y) dt dt
and the argument for drawdf
should be a list with the two
functions G and F, in that order; namely, the first
expression in the list will be taken to be the time derivative of the
variable represented on the horizontal axis, and the second expression
will be the time derivative of the variable represented on the vertical
axis. Those two variables do not have to be x and y, but if
they are not, then the second argument given to drawdf must be another
list naming the two variables, first the one on the horizontal axis and
then the one on the vertical axis.
If only one ODE is given, drawdf
will implicitly admit
x=t
, and G(x,y)=1
, transforming the non-autonomous
equation into a system of two autonomous equations.
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