/usr/share/doc/libsundials-serial/README is in libsundials-serial 2.5.0-3ubuntu3.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/share/doc/libsundials-serial/../libsundials-cvode1/README
/usr/share/doc/libsundials-serial/../libsundials-cvode1/README