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\section{The Fixed Points Window}\index{Fixed Points window}\index{window, Fixed Points}
%
%
\begin{figure}[H]
%\vspace{2.0in}
\centerline{\psfig{figure=ps/fixedpoints.ps,height=240pt}}
\caption{\label{Fixed Points}
The Fixed Points window.
}
\end{figure}
%
%
%
\begin{description}
\item[Window title:] DsTool: Fixed Points
\item[Function:] The Fixed Points window allows the
user to find equilibria and to compute one dimensional
stable and unstable manifolds.
\item[Description:] The Fixed Points window is opened by selecting the
Fixed Points option from the Panels menu button located in the Command
window. The window contains parameters for controlling the computation
of stable and unstable manifolds.
% The default configuration hides the
% text fields which exist below command buttons. These text fields
% are shown or hidden according to the value of the Settings setting.
Color coding\index{color, coding}\index{color, DsTool convention}
is performed according to the stability type of
objects: saddle points are marked as green crosses, sinks are
blue triangles, and sources are red squares. Furthermore,
unstable manifolds\index{color, manifolds} are plotted in red and stable manifolds
are plotted in blue.
%\item[Window type:] pop-up
%\item[Window attributes:] pinnable(in), non-resizable, fixed/variable template
\item[Panel items:] \mbox{}
\begin{itemize}
\item Fixed point algorithm stack setting:
Allows the user to choose an algorithm to use for root-finding.\index{root-finding algorithms} The
current options are the Secant method\index{secant method} and Newton's method\index{Newton's method}.
The default is Newton's method.
\item Seed algorithm stack setting:
Allows the user to choose the method of providing an initial guess (``seed'')
to the root-finding algorithm.
The choices are:
\begin{description}
\item[Monte Carlo:]\index{Monte Carlo method}
The seed is a pseudo-random point chosen from the volume in phase
space defined by the coordinate values in the Defaults
window. This is the default setting.
% If a secondary coordinate system such as polar
% coordinates is in use (as determined by the Coordinate
% System setting found in the Defaults window) then these
% secondary coordinates will be used to define the search region. \notimp{}
%
\item[Selected Point:]
The seed is the currently selected point.
The currently selected point is displayed in the
Selected Point window\index{window, Selected Point}.
\end{description}
\item Monte Carlo points numeric field:
Displays the number of guesses taken by the Monte Carlo routine. If the
Seed algorithm setting is Selected Point, then this field is not
relevant. The default value is 10.
\item Period numeric field:
Displays the integer length of periodic orbits to search for.
This field is only shown if the current dynamical system is a mapping.
Note that points with lower periods which
divide the specified period may also be located. The default value is 1.
Setting this field to a positive number should be used to
search for periodic points (for mappings), instead of using the Periodic Orbits window.
\item Found read-only text field:
Displays the number of periodic orbits or equilibrium points found by DsTool's fixed point
algorithms. This may be different than the total number of
fixed points stored in memory, since fixed points may be loaded into memory from a data file.
For a mapping, this field displays the number of {\em orbits} found, not the
total number of points found.
\item Fixed point algorithm parameters message:
Identifies the text items which follow as parameters associated with the finding of
fixed points.
\item maximum iterations numeric field:
Displays the maximum number of iterations allowed while trying to converge to a periodic point.
The default value is 30.
% See Variable Convergence below.
\item convergence criterion read-write text field:\index{convergence}
Displays the criterion used to determine when Newton's method has converged.
An iterative sequence is said to converge to a periodic point
(resp. equilibrium) if the norm of $f(x_i)-x_i$ (resp. $f(x_i)$)
is less than the value of this field. Here $f$ is some iterate
of the mapping (resp. $f$ defines the vector field).
The default value is $10^{-8}$.
\item minimum step read-write text field:
Displays the minimum step to take in Newton's method.
Periodic points (resp. equilibria) are found via
an iterative process. If the difference between successive points
is less than the value in this field, then no further steps will
be taken and the final iterative point will not be regarded as a periodic point.
The default value is $10^{-8}$.
\item duplicate criterion read-write text field:
Displays the distance used to determine when two points should be considered the same.
Two periodic points (resp. equilibria) are considered to be identical if the norm
of their difference is less than the entry in this field.
The default value is $10^{-6}$.
\item finite difference step read-write text field:
Displays the stepsize to use when using finite differencing in
the DsTool fixed point calculations. The default value is $10^{-6}$.
\item One-D manifold algorithm parameters message:
Identifies the text items which follow as parameters associated with the calculation
of one dimensional stable and unstable manifolds.
\item stable manifold steps numeric field:
Displays the number of time steps to take in computing the manifold.
The length of the stable manifold displayed depends on the strength of the eigenvalues and the
number of time steps taken along the manifolds.
The default value is 200. This number should
be increased for manifolds associated with small eigenvalues.
\item (stable manifold) points numeric field:
Displays the number of interpolating points between steps.
Given a stable manifold and its pre-image, the manifold may be better visualized by interpolating
additional points along the manifold. The default value is 1, but for
manifolds which oscillate wildly (\eg, the transverse intersection
of homoclinic orbits) this number can be chosen much larger.
\item unstable manifold steps numeric field:
Displays the number of time steps to take in computing the manifold.
The length of the unstable manifold displayed depends on the strength of the eigenvalues and the
number of time steps taken along the manifolds.
The default value is 200. This number should
be increased for manifolds associated with large eigenvalues.
\item (unstable manifold) points numeric field:
Displays the number of interpolating points between steps.
Given an unstable manifold and its pre-image, the manifold may be better visualized by interpolating
additional points along the manifold. The default value is 1, but for
manifolds which oscillate wildly (\eg, the transverse intersection
of homoclinic orbits) this number can be chosen much larger.
\item Initial stepsize read-write text field:
Displays the distance from the periodic point (resp. equilibrium)
along the eigenvector to choose the
initial point of the stable or unstable manifold calculation.
The default value
is $10^{-6}$. This number should be decreased for highly nonlinear systems,
\ie, when the linearization at a periodic point (resp. equilibria) is only
a good approximation to the (nonlinear) mapping (resp. vector field) within
a tiny neighborhood of the periodic point (resp. equilibria).
\item Find fixed points command button:
Initiates the search for equilibria, using the current window settings.
\item Compute 1-d manifolds command button:
Initiates the calculation and display of one dimensional
stable and unstable manifolds.
% \item 2-D Manifolds window button:
% Opens and brings to the foreground the 2-d Manifolds window.
\item Clear points command button:
Erases the periodic points in memory and resets the Found text
field to 0.
% The view windows will be refreshed to reflect the changes.
\item Clear manifolds command button:
Erases the manifolds computed by the Fixed Points window.
% The view windows are refreshed to reflect the changes.
\item Dismiss command button:
Closes the Fixed Points window.
% The fields will not revert to their default values.
\end{itemize}
\end{description}
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