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<refentry xmlns="http://docbook.org/ns/docbook" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:svg="http://www.w3.org/2000/svg" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:db="http://docbook.org/ns/docbook" xmlns:scilab="http://www.scilab.org" xml:lang="en" xml:id="aff2ab">
    <refnamediv>
        <refname>aff2ab</refname>
        <refpurpose>linear (affine) function to A,b conversion</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
        <title>Syntax</title>
        <synopsis>[A,b]=aff2ab(afunction,dimX,D [,flag])</synopsis>
    </refsynopsisdiv>
    <refsection>
        <title>Arguments</title>
        <variablelist>
            <varlistentry>
                <term>afunction</term>
                <listitem>
                    <para>
                        a scilab function <literal> Y =fct(X,D) </literal> where <literal>X, D, Y</literal> are <literal>list</literal> of matrices
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>dimX</term>
                <listitem>
                    <para>
                        a p x 2 integer matrix (<literal>p</literal> is the number of matrices in <literal>X</literal>)
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>D</term>
                <listitem>
                    <para>
                        a <literal>list</literal> of real matrices (or any other valid Scilab object).
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>flag</term>
                <listitem>
                    <para>
                        optional parameter (<literal>flag='f'</literal> or <literal>flag='sp'</literal>)
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>A</term>
                <listitem>
                    <para>a real matrix</para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>b</term>
                <listitem>
                    <para>
                        a real vector having same row dimension as <literal>A</literal>
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
    </refsection>
    <refsection>
        <title>Description</title>
        <para>
            <literal>aff2ab</literal>  returns the matrix representation of an affine
            function (in the canonical basis).
        </para>
        <para>
            <literal>afunction</literal> is a function with imposed syntax:
            <literal> Y=afunction(X,D) </literal> where <literal> X=list(X1,X2,...,Xp) </literal> is
            a list of p real matrices, and <literal> Y=list(Y1,...,Yq) </literal> is
            a list of q real real matrices which depend linearly of
            the <literal> Xi</literal>'s. The (optional) input <literal> D</literal> contains
            parameters needed to compute Y as a function of X.
            (It is generally a list of matrices).
        </para>
        <para>
            <literal> dimX</literal> is a p x 2 matrix: <literal>dimX(i)=[nri,nci]</literal>
            is the actual number of rows and columns of matrix <literal>Xi</literal>.
            These dimensions determine <literal>na</literal>, the column dimension of
            the resulting matrix <literal>A</literal>: <literal>na=nr1*nc1 +...+ nrp*ncp</literal>.
        </para>
        <para>
            If the optional parameter <literal>flag='sp'</literal> the resulting <literal>A</literal>
            matrix is returned as a sparse matrix.
        </para>
        <para>
            This function is useful to solve a system of linear equations
            where the unknown variables are matrices.
        </para>
    </refsection>
    <refsection>
        <title>Examples</title>
        <programlisting role="example"><![CDATA[
// Lyapunov equation solver (one unknown variable, one constraint)
deff('Y=lyapunov(X,D)','[A,Q]=D(:);Xm=X(:); Y=list(A''*Xm+Xm*A-Q)')
A=rand(3,3);Q=rand(3,3);Q=Q+Q';D=list(A,Q);dimX=[3,3];
[Aly,bly]=aff2ab(lyapunov,dimX,D);
[Xl,kerA]=linsolve(Aly,bly); Xv=vec2list(Xl,dimX); lyapunov(Xv,D)
Xm=Xv(:); A'*Xm+Xm*A-Q

// Lyapunov equation solver with redundant constraint X=X'
// (one variable, two constraints) D is global variable
deff('Y=ly2(X,D)','[A,Q]=D(:);Xm=X(:); Y=list(A''*Xm+Xm*A-Q,Xm''-Xm)')
A=rand(3,3);Q=rand(3,3);Q=Q+Q';D=list(A,Q);dimX=[3,3];
[Aly,bly]=aff2ab(ly2,dimX,D);
[Xl,kerA]=linsolve(Aly,bly); Xv=vec2list(Xl,dimX); ly2(Xv,D)

// Francis equations
// Find matrices X1 and X2 such that:
// A1*X1 - X1*A2 + B*X2 -A3 = 0
// D1*X1 -D2 = 0
deff('Y=bruce(X,D)','[A1,A2,A3,B,D1,D2]=D(:);'+...
'[X1,X2]=X(:);Y=list(A1*X1-X1*A2+B*X2-A3,D1*X1-D2)')
A1=[-4,10;-1,2];A3=[1;2];B=[0;1];A2=1;D1=[0,1];D2=1;
D=list(A1,A2,A3,B,D1,D2);
[n1,m1]=size(A1);[n2,m2]=size(A2);[n3,m3]=size(B);
dimX=[[m1,n2];[m3,m2]];
[Af,bf]=aff2ab(bruce,dimX,D);
[Xf,KerAf]=linsolve(Af,bf);Xsol=vec2list(Xf,dimX)
bruce(Xsol,D)

// Find all X which commute with A
deff('y=f(X,D)','y=list(D(:)*X(:)-X(:)*D(:))')
A=rand(3,3);dimX=[3,3];[Af,bf]=aff2ab(f,dimX,list(A));
[Xf,KerAf]=linsolve(Af,bf);[p,q]=size(KerAf);
Xsol=vec2list(Xf+KerAf*rand(q,1),dimX);
C=Xsol(:); A*C-C*A
 ]]></programlisting>
    </refsection>
    <refsection role="see also">
        <title>See also</title>
        <simplelist type="inline">
            <member>
                <link linkend="linsolve">linsolve</link>
            </member>
        </simplelist>
    </refsection>
</refentry>
