规范模块目录和第三方依赖结构

third_party/eigen3/doc/snippets/Tridiagonalization_decomposeInPlace.cpp

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+MatrixXd X = MatrixXd::Random(5,5);
+MatrixXd A = X + X.transpose();
+cout << "Here is a random symmetric 5x5 matrix:" << endl << A << endl << endl;
+
+VectorXd diag(5);
+VectorXd subdiag(4);
+internal::tridiagonalization_inplace(A, diag, subdiag, true);
+cout << "The orthogonal matrix Q is:" << endl << A << endl;
+cout << "The diagonal of the tridiagonal matrix T is:" << endl << diag << endl;
+cout << "The subdiagonal of the tridiagonal matrix T is:" << endl << subdiag << endl;