/usr/include/ql/math/interpolations/ is in libquantlib0-dev 1.7.1-1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/ql/math/interpolations/abcdinterpolation.hpp
/usr/include/ql/math/interpolations/all.hpp
/usr/include/ql/math/interpolations/backwardflatinterpolation.hpp
/usr/include/ql/math/interpolations/backwardflatlinearinterpolation.hpp
/usr/include/ql/math/interpolations/bicubicsplineinterpolation.hpp
/usr/include/ql/math/interpolations/bilinearinterpolation.hpp
/usr/include/ql/math/interpolations/convexmonotoneinterpolation.hpp
/usr/include/ql/math/interpolations/cubicinterpolation.hpp
/usr/include/ql/math/interpolations/extrapolation.hpp
/usr/include/ql/math/interpolations/flatextrapolation2d.hpp
/usr/include/ql/math/interpolations/forwardflatinterpolation.hpp
/usr/include/ql/math/interpolations/interpolation2d.hpp
/usr/include/ql/math/interpolations/kernelinterpolation.hpp
/usr/include/ql/math/interpolations/kernelinterpolation2d.hpp
/usr/include/ql/math/interpolations/linearinterpolation.hpp
/usr/include/ql/math/interpolations/loginterpolation.hpp
/usr/include/ql/math/interpolations/mixedinterpolation.hpp
/usr/include/ql/math/interpolations/multicubicspline.hpp
/usr/include/ql/math/interpolations/sabrinterpolation.hpp
/usr/include/ql/math/interpolations/xabrinterpolation.hpp