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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="im_inv">
    <refnamediv>
        <refname>im_inv</refname>
        <refpurpose>inverse image</refpurpose>
    </refnamediv>
    <refsynopsisdiv>
        <title>Syntax</title>
        <synopsis>[X,dim]=im_inv(A,B [,tol])
            [X,dim,Y]=im_inv(A,B, [,tol])
        </synopsis>
    </refsynopsisdiv>
    <refsection>
        <title>Arguments</title>
        <variablelist>
            <varlistentry>
                <term>A,B</term>
                <listitem>
                    <para>two real or complex matrices with equal number of columns</para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>X</term>
                <listitem>
                    <para>
                        orthogonal or unitary square matrix of order equal to the  number of columns of <literal>A</literal>
                    </para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>dim</term>
                <listitem>
                    <para>integer (dimension of subspace)</para>
                </listitem>
            </varlistentry>
            <varlistentry>
                <term>Y</term>
                <listitem>
                    <para>
                        orthogonal matrix of order equal to the number of rows  of <literal>A</literal> and <literal>B</literal>.
                    </para>
                </listitem>
            </varlistentry>
        </variablelist>
    </refsection>
    <refsection>
        <title>Description</title>
        <para>
            <literal>[X,dim]=im_inv(A,B)</literal> computes <literal>(A^-1)(B)</literal>
            i.e vectors whose image through <literal>A</literal> are in
            range(<literal>B</literal>)
        </para>
        <para>
            The <literal>dim</literal> first columns of <literal>X</literal> span
            <literal>(A^-1)(B)</literal>
        </para>
        <para>
            <literal>tol</literal> is a threshold used to test if subspace inclusion;
            default value is <literal>tol = 100*%eps</literal>.
            If <literal>Y</literal> is returned, then <literal>[Y*A*X,Y*B]</literal> is partitioned as follows:
            <literal>[A11,A12;0,A22]</literal>,<literal>[B1;0]</literal>
        </para>
        <para>
            where <literal>B1</literal> has full row rank (equals
            <literal>rank(B)</literal>) and <literal>A22</literal> has full column rank
            and has <literal>dim</literal> columns.
        </para>
    </refsection>
    <refsection>
        <title>Examples</title>
        <programlisting role="example"><![CDATA[
A=[rand(2,5);[zeros(3,4),rand(3,1)]];B=[[1,1;1,1];zeros(3,2)];
W=rand(5,5);A=W*A;B=W*B;
[X,dim]=im_inv(A,B)
svd([A*X(:,1:dim),B])   //vectors A*X(:,1:dim) belong to range(B)
[X,dim,Y]=im_inv(A,B);[Y*A*X,Y*B]
 ]]></programlisting>
    </refsection>
    <refsection role="see also">
        <title>See also</title>
        <simplelist type="inline">
            <member>
                <link linkend="rowcomp">rowcomp</link>
            </member>
            <member>
                <link linkend="spaninter">spaninter</link>
            </member>
            <member>
                <link linkend="spanplus">spanplus</link>
            </member>
            <member>
                <link linkend="linsolve">linsolve</link>
            </member>
        </simplelist>
    </refsection>
</refentry>
