From: bangerth Date: Wed, 29 Jun 2011 18:22:18 +0000 (+0000) Subject: Add a note to a comment. X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=3a853ec23d1658ccc0c57510025e460219c31063;p=dealii-svn.git Add a note to a comment. git-svn-id: https://svn.dealii.org/trunk@23880 0785d39b-7218-0410-832d-ea1e28bc413d --- diff --git a/deal.II/include/deal.II/numerics/vectors.h b/deal.II/include/deal.II/numerics/vectors.h index f18e6e5f1b..f1b4532938 100644 --- a/deal.II/include/deal.II/numerics/vectors.h +++ b/deal.II/include/deal.II/numerics/vectors.h @@ -1,7 +1,7 @@ //--------------------------------------------------------------------------- // $Id$ // -// Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 by the deal.II authors +// Copyright (C) 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 by the deal.II authors // // This file is subject to QPL and may not be distributed // without copyright and license information. Please refer @@ -1949,7 +1949,7 @@ class VectorTools * called in a preconditioner and * generates updates with mean value * zero. The mean value is computed as - * the mean value of the degree of + * the mean value of the degrees of * freedom values as given by the input * vector; they are not weighted by the * area of cells, i.e. the mean is @@ -1961,8 +1961,8 @@ class VectorTools * however. * * Apart from the vector @p v to operate - * on, this function takes a bit - * vector. This has a true entry for + * on, this function takes a boolean mask + * that has a true entry for * every component for which the mean * value shall be computed and later * subtracted. The argument is used to @@ -1971,6 +1971,34 @@ class VectorTools * pressure, and avoid touching all other * components of the vector, such as the * velocity components. + * + * @note In the context of using this + * function to filter out the kernel of + * an operator (such as the null space of + * the Stokes operator that consists of + * the constant pressures), this function + * only makes sense for finite elements + * for which the null space indeed + * consists of the vector + * $(1,1,\ldots,1)^T$. This is the case + * for example for the usual Lagrange + * elements where the sum of all shape + * functions equals the function that is + * constant one. However, it is not true + * for some other functions: for example, + * for the FE_DGP element (another valid + * choice for the pressure in Stokes + * discretizations), the first shape + * function on each cell is constant + * while further elements are $L_2$ + * orthogonal to it (on the reference + * cell); consequently, the sum of all + * shape functions is not equal to one, + * and the vector that is associated with + * the constant mode is not equal to + * $(1,1,\ldots,1)^T$. For such elements, + * a different procedure has to be used + * when subtracting the mean value. */ static void subtract_mean_value(Vector &v, const std::vector &p_select);