Abstract
A multigrid method for linear systems stemming from the Galerkin B-spline discretization of classical second-order elliptic problems is considered. The spectral features of the involved stiffness matrices, as the fineness parameter h tends to zero, have been deeply studied in previous works, with particular attention to the dependencies of the spectrum on the degree p of the B-splines used in the discretization process. Here, by exploiting this information in connection with \(\tau \)-matrices, we describe a multigrid strategy and we prove that the corresponding two-grid iterations have a convergence rate independent of h for \(p=1,2,3\). For larger p, the proof may be obtained through algebraic manipulations. Unfortunately, as confirmed by the numerical experiments, the dependence on p is bad and hence other techniques have to be considered for large p.

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Notes
Take into account that these statements hold when the discretization steps \(h_i=1/n_i\) in each direction \(x_i\) tend to 0 with the same speed. This happens, for instance, when \(n_i=\nu _in,\ \varvec{\nu }:=(\nu _1,\ldots ,\nu _d)\) is fixed and \(n\rightarrow \infty \).
The first property holds because \(q_d\) is nonnegative and, by a direct computation,
$$\begin{aligned} \sum \nolimits _{{\widehat{{\varvec{\theta }}}}\in {\mathcal {M}}({\varvec{\theta }})\cup \{{\varvec{\theta }}\}}q_d({\widehat{{\varvec{\theta }}}})=2^d>0,\quad \forall {\varvec{\theta }}\in [0,\pi ]^d. \end{aligned}$$
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Acknowledgments
This work was partially supported by INdAM-GNCS Gruppo Nazionale per il Calcolo Scientifico, by the MIUR ‘Futuro in Ricerca 2013’ Programme through the project DREAMS, by the MIUR-PRIN 2012 N. 2012MTE38N, and by the Donation KAW 2013.0341 from the Knut & Alice Wallenberg Foundation in collaboration with the Royal Swedish Academy of Sciences, supporting Swedish research in mathematics.
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Communicated by Artem Napov, Yvan Notay, and Stefan Vandewalle.
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Donatelli, M., Garoni, C., Manni, C. et al. Two-grid optimality for Galerkin linear systems based on B-splines. Comput. Visual Sci. 17, 119–133 (2015). https://doi.org/10.1007/s00791-015-0253-z
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DOI: https://doi.org/10.1007/s00791-015-0253-z