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third_party/eigen/unsupported/doc/examples/MatrixFunction.cpp

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+#include <unsupported/Eigen/MatrixFunctions>
+#include <iostream>
+
+using namespace Eigen;
+
+std::complex<double> expfn(std::complex<double> x, int) { return std::exp(x); }
+
+int main() {
+  const double pi = std::acos(-1.0);
+
+  MatrixXd A(3, 3);
+  A << 0, -pi / 4, 0, pi / 4, 0, 0, 0, 0, 0;
+
+  std::cout << "The matrix A is:\n" << A << "\n\n";
+  std::cout << "The matrix exponential of A is:\n" << A.matrixFunction(expfn) << "\n\n";
+}