/usr/include/ql/methods/finitedifferences/solvers/ is in libquantlib0-dev 1.4-2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ql/methods/finitedifferences/solvers/all.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdm1dimsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdm2dblackscholessolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdm2dimsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdm3dimsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmbackwardsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmbatessolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmblackscholessolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmg2solver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmhestonhullwhitesolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmhestonsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmhullwhitesolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmndimsolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmsimple2dbssolver.hpp
/usr/include/ql/methods/finitedifferences/solvers/fdmsolverdesc.hpp