The path where apt-get installs eigen3 is /usr/include/eigen3; in addition, you need to use the following command to map Eigen to the /usr/include path, because in many programs you often use #include <Eigen/Dense> instead Use #include <eigen3/Eigen/Dense>. If you don’t map, some programs will report an error because Eigen/Dense cannot be found when compiling.
sudo cp -r /usr/local/include/eigen3 /usr/include/eigen3
sudo ln -s /usr/include/eigen3/Eigen /usr/include/Eigen
So far you're done, and now you install Ceres and you won't get an error.