Polyhedron/external/eigen-3.4.0/doc/snippets/LeastSquaresNormalEquations.cpp

MatrixXf A = MatrixXf::Random(3, 2);
VectorXf b = VectorXf::Random(3);
cout << "The solution using normal equations is:\n"
     << (A.transpose() * A).ldlt().solve(A.transpose() * b) << endl;