Add Spectra.

thirdparty/include/Spectra/Util/GEigsMode.h

@@ -0,0 +1,28 @@
+// Copyright (C) 2016-2022 Yixuan Qiu <yixuan.qiu@cos.name>
+//
+// This Source Code Form is subject to the terms of the Mozilla
+// Public License v. 2.0. If a copy of the MPL was not distributed
+// with this file, You can obtain one at https://mozilla.org/MPL/2.0/.
+
+#ifndef SPECTRA_GEIGS_MODE_H
+#define SPECTRA_GEIGS_MODE_H
+
+namespace Spectra {
+
+///
+/// \ingroup Enumerations
+///
+/// The enumeration to specify the mode of generalized eigenvalue solver.
+///
+enum class GEigsMode
+{
+    Cholesky,        ///< Using Cholesky decomposition to solve generalized eigenvalues.
+    RegularInverse,  ///< Regular inverse mode for generalized eigenvalue solver.
+    ShiftInvert,     ///< Shift-and-invert mode for generalized eigenvalue solver.
+    Buckling,        ///< Buckling mode for generalized eigenvalue solver.
+    Cayley           ///< Cayley transformation mode for generalized eigenvalue solver.
+};
+
+}  // namespace Spectra
+
+#endif  // SPECTRA_GEIGS_MODE_H