/usr/lib/x86_64-linux-gnu/libSimTKmath.so is in libsimbody-dev 3.5.3+dfsg-1ubuntu1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/lib/x86_64-linux-gnu/libSimTKmath.so.3.5
/usr/lib/x86_64-linux-gnu/libSimTKmath.so.3.5