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<title>GNU Scientific Library &ndash; Reference Manual: Pivoted Cholesky Decomposition</title>

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<a name="Pivoted-Cholesky-Decomposition"></a>
<div class="header">
<p>
Next: <a href="Modified-Cholesky-Decomposition.html#Modified-Cholesky-Decomposition" accesskey="n" rel="next">Modified Cholesky Decomposition</a>, Previous: <a href="Cholesky-Decomposition.html#Cholesky-Decomposition" accesskey="p" rel="previous">Cholesky Decomposition</a>, Up: <a href="Linear-Algebra.html#Linear-Algebra" accesskey="u" rel="up">Linear Algebra</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
</div>
<hr>
<a name="Pivoted-Cholesky-Decomposition-1"></a>
<h3 class="section">14.7 Pivoted Cholesky Decomposition</h3>
<a name="index-Cholesky-decomposition_002c-pivoted"></a>
<a name="index-Pivoted-Cholesky-Decomposition"></a>

<p>A symmetric, positive definite square matrix <em>A</em> has an alternate
Cholesky decomposition into a product of a lower unit triangular matrix <em>L</em>,
a diagonal matrix <em>D</em> and <em>L^T</em>, given by <em>L D L^T</em>. This is
equivalent to the Cholesky formulation discussed above, with
the standard Cholesky lower triangular factor given by <em>L D^{1 \over 2}</em>.
For ill-conditioned matrices, it can help to use a pivoting strategy to
prevent the entries of <em>D</em> and <em>L</em> from growing too large, and also
ensure <em>D_1 \ge D_2 \ge \cdots \ge D_n &gt; 0</em>, where <em>D_i</em> are
the diagonal entries of <em>D</em>. The final decomposition is given by
</p>
<div class="example">
<pre class="example">P A P^T = L D L^T
</pre></div>

<p>where <em>P</em> is a permutation matrix.
</p>
<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fdecomp"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_decomp</strong> <em>(gsl_matrix * <var>A</var>, gsl_permutation * <var>p</var>)</em></dt>
<dd><p>This function factors the symmetric, positive-definite square matrix
<var>A</var> into the Pivoted Cholesky decomposition <em>P A P^T = L D L^T</em>.
On input, the values from the diagonal and lower-triangular part of the matrix <var>A</var> are
used to construct the factorization. On output the diagonal of the input matrix <var>A</var> stores
the diagonal elements of <em>D</em>, and the lower triangular portion of <var>A</var>
contains the matrix <em>L</em>. Since <em>L</em> has ones on its diagonal these
do not need to be explicitely stored. The upper triangular portion of <var>A</var> is
unmodified. The permutation matrix <em>P</em> is stored in <var>p</var> on output.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fsolve"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_solve</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, const gsl_vector * <var>b</var>, gsl_vector * <var>x</var>)</em></dt>
<dd><p>This function solves the system <em>A x = b</em> using the Pivoted Cholesky
decomposition of <em>A</em> held in the matrix <var>LDLT</var> and permutation
<var>p</var> which must have been previously computed by <code>gsl_linalg_pcholesky_decomp</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fsvx"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_svx</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, gsl_vector * <var>x</var>)</em></dt>
<dd><p>This function solves the system <em>A x = b</em> in-place using the Pivoted Cholesky
decomposition of <em>A</em> held in the matrix <var>LDLT</var> and permutation
<var>p</var> which must have been previously computed by <code>gsl_linalg_pcholesky_decomp</code>.
On input, <var>x</var> contains the right hand side vector <em>b</em> which is
replaced by the solution vector on output.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fdecomp2"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_decomp2</strong> <em>(gsl_matrix * <var>A</var>, gsl_permutation * <var>p</var>, gsl_vector * <var>S</var>)</em></dt>
<dd><p>This function computes the pivoted Cholesky factorization of the matrix
<em>S A S</em>, where the input matrix <var>A</var> is symmetric and positive
definite, and the diagonal scaling matrix <var>S</var> is computed to reduce the
condition number of <var>A</var> as much as possible. See
<a href="Cholesky-Decomposition.html#Cholesky-Decomposition">Cholesky Decomposition</a> for more information on the matrix <var>S</var>.
The Pivoted Cholesky decomposition satisfies <em>P S A S P^T = L D L^T</em>.
On input, the values from the diagonal and lower-triangular part of the matrix <var>A</var> are
used to construct the factorization.  On output the diagonal of the input matrix <var>A</var> stores the diagonal
elements of <em>D</em>, and the lower triangular portion of <var>A</var>
contains the matrix <em>L</em>. Since <em>L</em> has ones on its diagonal these
do not need to be explicitely stored. The upper triangular portion of <var>A</var>
is unmodified. The permutation matrix <em>P</em> is stored in <var>p</var> on output.
The diagonal scaling transformation is stored in <var>S</var> on output.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fsolve2"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_solve2</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, const gsl_vector * <var>S</var>, const gsl_vector * <var>b</var>, gsl_vector * <var>x</var>)</em></dt>
<dd><p>This function solves the system <em>(S A S) (S^{-1} x) = S b</em> using the Pivoted Cholesky
decomposition of <em>S A S</em> held in the matrix <var>LDLT</var>, permutation
<var>p</var>, and vector <var>S</var>, which must have been previously computed by
<code>gsl_linalg_pcholesky_decomp2</code>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005fsvx2"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_svx2</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, const gsl_vector * <var>S</var>, gsl_vector * <var>x</var>)</em></dt>
<dd><p>This function solves the system <em>(S A S) (S^{-1} x) = S b</em> in-place using the Pivoted Cholesky
decomposition of <em>S A S</em> held in the matrix <var>LDLT</var>, permutation
<var>p</var> and vector <var>S</var>, which must have been previously computed by
<code>gsl_linalg_pcholesky_decomp2</code>.
On input, <var>x</var> contains the right hand side vector <em>b</em> which is
replaced by the solution vector on output.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005finvert"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_invert</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, gsl_matrix * <var>Ainv</var>)</em></dt>
<dd><p>This function computes the inverse of the matrix <em>A</em>, using the Pivoted
Cholesky decomposition stored in <var>LDLT</var> and <var>p</var>. On output, the
matrix <var>Ainv</var> contains <em>A^{-1}</em>.
</p></dd></dl>

<dl>
<dt><a name="index-gsl_005flinalg_005fpcholesky_005frcond"></a>Function: <em>int</em> <strong>gsl_linalg_pcholesky_rcond</strong> <em>(const gsl_matrix * <var>LDLT</var>, const gsl_permutation * <var>p</var>, double * <var>rcond</var>, gsl_vector * <var>work</var>)</em></dt>
<dd><p>This function estimates the reciprocal condition number (using the 1-norm) of the symmetric positive
definite matrix <em>A</em>, using its pivoted Cholesky decomposition provided in <var>LDLT</var>.
The reciprocal condition number estimate, defined as <em>1 / (||A||_1 \cdot ||A^{-1}||_1)</em>, is stored
in <var>rcond</var>.  Additional workspace of size <em>3 N</em> is required in <var>work</var>.
</p></dd></dl>

<hr>
<div class="header">
<p>
Next: <a href="Modified-Cholesky-Decomposition.html#Modified-Cholesky-Decomposition" accesskey="n" rel="next">Modified Cholesky Decomposition</a>, Previous: <a href="Cholesky-Decomposition.html#Cholesky-Decomposition" accesskey="p" rel="previous">Cholesky Decomposition</a>, Up: <a href="Linear-Algebra.html#Linear-Algebra" accesskey="u" rel="up">Linear Algebra</a> &nbsp; [<a href="Function-Index.html#Function-Index" title="Index" rel="index">Index</a>]</p>
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