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<div id="version" align=right><b>petsc-3.14.5 2021-03-03</b></div>
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<A NAME="SNESNGS"><H1>SNESNGS</H1></A>
Either calls the user-provided solution routine provided with <A HREF="../SNES/SNESSetNGS.html#SNESSetNGS">SNESSetNGS</A>() or does a finite difference secant approximation using coloring.
<P>
<H3><FONT COLOR="#CC3333">Options Database</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_sweeps <n> </B></TD><TD>- Number of sweeps of GS to apply
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_atol <atol> </B></TD><TD>- Absolute residual tolerance for GS iteration
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_rtol <rtol> </B></TD><TD>- Relative residual tolerance for GS iteration
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_stol <stol> </B></TD><TD>- Absolute update tolerance for GS iteration
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_max_it <maxit> </B></TD><TD>- Maximum number of sweeps of GS to apply
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_secant </B></TD><TD>- Use pointwise secant local Jacobian approximation with coloring instead of user provided Gauss-Seidel routine, this is
used by default if no user provided Gauss-Seidel routine is available. Requires either that a <A HREF="../DM/DM.html#DM">DM</A> that can compute a coloring
is available or a Jacobian sparse matrix is provided (from which to get the coloring).
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_secant_h <h> </B></TD><TD>- Differencing parameter for secant approximation
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_ngs_secant_mat_coloring </B></TD><TD>- Use the graph coloring of the Jacobian for the secant GS even if a <A HREF="../DM/DM.html#DM">DM</A> is available.
</TD></TR>
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>-snes_norm_schedule <none, always, initialonly, finalonly, initalfinalonly> </B></TD><TD>- how often the residual norms are computed
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">Notes</FONT></H3>
the Gauss-Seidel smoother is inherited through composition. If a solver has been created with <A HREF="../SNES/SNESGetNPC.html#SNESGetNPC">SNESGetNPC</A>(), it will have
its parent's Gauss-Seidel routine associated with it.
<P>
By default this routine computes the solution norm at each iteration, this can be time consuming, you can turn this off with <A HREF="../SNES/SNESSetNormSchedule.html#SNESSetNormSchedule">SNESSetNormSchedule</A>() or -snes_norm_schedule
<H3><FONT COLOR="#CC3333">References</FONT></H3>
<TABLE border="0" cellpadding="0" cellspacing="0">
<TR><TD WIDTH=40></TD><TD ALIGN=LEFT VALIGN=TOP><B>1. </B></TD><TD>- Peter R. Brune, Matthew G. Knepley, Barry F. Smith, and Xuemin Tu, "Composing Scalable Nonlinear Algebraic Solvers",
SIAM Review, 57(4), 2015
</TD></TR></TABLE>
<P>
<H3><FONT COLOR="#CC3333">See Also</FONT></H3>
<A HREF="../SNES/SNESCreate.html#SNESCreate">SNESCreate</A>(), <A HREF="../SNES/SNES.html#SNES">SNES</A>, <A HREF="../SNES/SNESSetType.html#SNESSetType">SNESSetType</A>(), <A HREF="../SNES/SNESSetNGS.html#SNESSetNGS">SNESSetNGS</A>(), <A HREF="../SNES/SNESType.html#SNESType">SNESType</A> (for list of available types), <A HREF="../SNES/SNESNGSSetSweeps.html#SNESNGSSetSweeps">SNESNGSSetSweeps</A>(), <A HREF="../SNES/SNESNGSSetTolerances.html#SNESNGSSetTolerances">SNESNGSSetTolerances</A>(),
<BR><A HREF="../SNES/SNESSetNormSchedule.html#SNESSetNormSchedule">SNESSetNormSchedule</A>()
<P><B></B><H3><FONT COLOR="#CC3333">Level</FONT></H3>advanced<BR>
<H3><FONT COLOR="#CC3333">Location</FONT></H3>
</B><A HREF="../../../src/snes/impls/gs/snesgs.c.html#SNESNGS">src/snes/impls/gs/snesgs.c</A>
<BR><A HREF="./index.html">Index of all SNES routines</A>
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