# Singular

Procedure from library `finitediff.lib` (see finitediff_lib).

Usage:

Return:
list

Purpose:
gives back some constraints that are equivalent to the eigenvalues of the matrices in the list l being < 1 (computed by QEPCAD)

Example:
 ```LIB "finitediff.lib"; list D="Ut","Ux","Uy","U"; list V="t","x","y"; list P="a","b"; setinitials(V,D,P); matrix M[2][2]=0,-a,-a,0; list Mat=unitmat(2),M; list Appr=forward(Ut,U,t),forward(Ux,U,x); //matrixsystem(Mat,Appr); //timestep(_); fouriersystem(Mat,Appr); ==> [1]: ==> [1]: ==> [1]: ==> _[1,1]=(T) ==> _[1,2]=0 ==> _[2,1]=0 ==> _[2,2]=(T) ==> [2]: ==> _[1,1]=1 ==> _[1,2]=(I*Sx*a*dt+Cx*a*dt-a*dt)/(dx) ==> _[2,1]=(I*Sx*a*dt+Cx*a*dt-a*dt)/(dx) ==> _[2,2]=1 ==> [2]: ==> [1]: ==> _[1]=(T) ==> [2]: ==> _[1]=(-I*Sx*a*dt-Cx*a*dt+a*dt+dx)/(dx) ==> _[2]=(I*Sx*a*dt+Cx*a*dt-a*dt+dx)/(dx) ==> [2]: ==> [1]: ==> [1]: ==> (T^2) ==> [2]: ==> [1]: ==> (-2*Cx*a^2*dt^2-2*Cx*a*dt*dx+2*a^2*dt^2+2*a*dt*dx+dx^2)/(dx^2) ==> [2]: ==> (-2*Cx*a^2*dt^2+2*Cx*a*dt*dx+2*a^2*dt^2-2*a*dt*dx+dx^2)/(dx^2) qepcadsystem(_[2]); ==> 0 ==> 0 ==> 0 ==> sh: qepcad: Kommando nicht gefunden. ==> sh: qepcadfilter.pl: Kommando nicht gefunden. ==> 32512 ==> ==> ? Try manually ==> ? leaving finitediff.lib::qepcadsystem ```