Radial basis function-generated finite differences (RBF-FD)

This page describes the computation of RBF-FD weight augmented with polynomials. See also Computation of shape functions and Meshless Local Strong Form Method (MLSM) for a more general discussion. Approximations of partial differential operators are the core of strong form meshless procedures. Consider a partial differential operator $\L$ at a point $\x_c$. Approximation of $\L$ at a point $\x_c$ is sought using an ansatz \begin{equation} \label{eq:approx} (\L u)(\x_c) \approx \sum_{i=1}^{n} w_i u(\x_i). \end{equation} Here $\x_i$ are the neighboring nodes of $\x_c$ which constitute its \emph{stencil}, $w_i$ are called \emph{stencil weights}, $n$ is the \emph{stencil size} and $u$ is an arbitrary function.

This form of approximation is desirable, since operator $\L$ at point $\x_c$ is approximated by a linear functional $\w_\L (\x_c)$, assembled of weights $\w_i$ \begin{equation} \label{eq:approx-vec} \L|_{\x_c} \approx \w_\L (\x_c)^\T \end{equation} and the approximation is obtained using just a dot product with the function values in neighboring nodes. The dependence of $\w_\L (\x_c)$ on $\L$ and $\x_c$ is often omitted and written simply as $\w$.

To determine the unknown weights $\w$, equality of~\eqref{eq:approx} is enforced for a given set of functions. A natural choice are monomials, which are also used in FDM, resulting in the Finite Point Method~\cite{onate2001finite}.

In the RBF-FD discretization the equality is satisfied for radial basis functions $\phi_j$. Each $\phi_j$, for $j = 1, \ldots, n$ corresponds to one linear equation \begin{equation} \sum_{i=1}^{n} w_i \phi_j (\x_i) = (\L \phi_j)(\x_c) \end{equation} for unknowns $w_i$. Assembling these $n$ equations for into matrix form, we obtain the following linear system: \begin{equation} \label{eq:rbf-system} \begin{bmatrix} \phi(\|\x_1 - \x_1\|) &\cdots & \phi(\|\x_n - \x_1\|) \\ \vdots & \ddots & \vdots \\ \phi(\|\x_1 - \x_n\|) &\cdots & \phi(\|\x_n - \x_n\|) \end{bmatrix} \begin{bmatrix} w_1 \\ \vdots \\ w_n \end{bmatrix} = \begin{bmatrix} (\L\phi(\|\x-\x_1\|))|_{\x=\x_c} \\ \vdots \\ (\L\phi(\|\x-\x_n\|))|_{\x=\x_c} \\ \end{bmatrix}, \end{equation} where $\phi_j$ have been expanded for clarity. The above system will be written more compactly as \begin{equation} \label{eq:rbf-system-c} A \w = \b \ell_\phi. \end{equation} The matrix $A$ is symmetric, and for some $\phi$ even positive definite. This and other approximation properties of RBFs are well studied and out of scope of this work~\cite{wendland2004scattered}.

\subsection{Monomial augmentation} \label{sec:aug} Using approximations that only contain RBF can lead to stability issues with conditioning under refinement or failure to converge due to stagnation errors~\cite{flyer2016role}. To improve accuracy and convergence, the approximation from section~\ref{sec:rbffd} can be augmented with polynomials.

Let $p_1, \ldots, p_s$ be polynomials forming the basis of the space of $d$-dimensional multivariate polynomials up to and including total degree $m$, with $s = \binom{m+d}{d}$.

Since enforcing exactness of~\eqref{eq:approx} for additional function would result in an overdetermined system, these additional constraints are enforced by extending~\eqref{eq:rbf-system-c} as \begin{equation} \label{eq:rbf-system-aug} \begin{bmatrix} A & P \\ P^\T & 0 \end{bmatrix} \begin{bmatrix} \w \\ \b \lambda \end{bmatrix} = \begin{bmatrix} \b \ell_{\phi} \\ \b \ell_{p} \end{bmatrix}\!\!,\ P = \begin{bmatrix} p_1(\x_1) & \cdots & p_s(\x_1) \\ \vdots & \ddots & \vdots \\ p_1(\x_n) & \cdots & p_s(\x_n) \\ \end{bmatrix}\!\!,\ \b \ell_p = \begin{bmatrix} (\L p_1)|_{\x=\x_c} \\ \vdots \\ (\L p_s)|_{\x=\x_c} \\ \end{bmatrix} \end{equation} where $P$ is a $n \times s$ matrix of polynomials evaluated at stencil nodes and $\b \ell_p$ is the vector of values assembled by applying considered operator $\L$ to the polynomials at $\x_c$.

Note that the equation $P^\T \w = \b \ell_p$ contains exactly exactness constraints for $p_j$. However, the introduction of parameters $\lambda_j$ causes~\eqref{eq:approx} to not be exact for $\phi_i$ anymore. In fact, is was shown~\cite{flyer2016role} to be equivalent to the following constrained minimisation problem \begin{equation} \min_{\w} \left(\frac{1}{2} \w^\T A \w - \w^\T \b \ell_{\phi}\right), \text{ subject to } P^\T \b w = \ell_{p} \end{equation} and parameters $\b \lambda$ can be interpreted as Lagrangian multipliers.

Weights obtained by solving~\eqref{eq:rbf-system-aug} are taken as approximations of $\L$ at $\x_c$, while values $\b \lambda$ are discarded.