/usr/include/simbody/simmath/internal/ is in libsimbody-dev 3.5.4+dfsg-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/simbody/simmath/internal/BicubicSurface.h
/usr/include/simbody/simmath/internal/CollisionDetectionAlgorithm.h
/usr/include/simbody/simmath/internal/Contact.h
/usr/include/simbody/simmath/internal/ContactGeometry.h
/usr/include/simbody/simmath/internal/ContactTracker.h
/usr/include/simbody/simmath/internal/GCVSPLUtil.h
/usr/include/simbody/simmath/internal/Geo.h
/usr/include/simbody/simmath/internal/Geo_BicubicBezierPatch.h
/usr/include/simbody/simmath/internal/Geo_BicubicHermitePatch.h
/usr/include/simbody/simmath/internal/Geo_Box.h
/usr/include/simbody/simmath/internal/Geo_CubicBezierCurve.h
/usr/include/simbody/simmath/internal/Geo_CubicHermiteCurve.h
/usr/include/simbody/simmath/internal/Geo_LineSeg.h
/usr/include/simbody/simmath/internal/Geo_Point.h
/usr/include/simbody/simmath/internal/Geo_Sphere.h
/usr/include/simbody/simmath/internal/Geo_Triangle.h
/usr/include/simbody/simmath/internal/Geodesic.h
/usr/include/simbody/simmath/internal/GeodesicIntegrator.h
/usr/include/simbody/simmath/internal/OBBTree.h
/usr/include/simbody/simmath/internal/OptimizerRep.h
/usr/include/simbody/simmath/internal/OrientedBoundingBox.h
/usr/include/simbody/simmath/internal/ParticleConSurfaceSystem.h
/usr/include/simbody/simmath/internal/SimTKcpodes.h
/usr/include/simbody/simmath/internal/Spline.h
/usr/include/simbody/simmath/internal/SplineFitter.h
/usr/include/simbody/simmath/internal/common.h