/usr/include/simbody/simmath/ is in libsimbody-dev 3.5.4+dfsg-1ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/simbody/simmath/CPodesIntegrator.h
/usr/include/simbody/simmath/Differentiator.h
/usr/include/simbody/simmath/ExplicitEulerIntegrator.h
/usr/include/simbody/simmath/Integrator.h
/usr/include/simbody/simmath/LinearAlgebra.h
/usr/include/simbody/simmath/MultibodyGraphMaker.h
/usr/include/simbody/simmath/Optimizer.h
/usr/include/simbody/simmath/RungeKutta2Integrator.h
/usr/include/simbody/simmath/RungeKutta3Integrator.h
/usr/include/simbody/simmath/RungeKuttaFeldbergIntegrator.h
/usr/include/simbody/simmath/RungeKuttaMersonIntegrator.h
/usr/include/simbody/simmath/SemiExplicitEuler2Integrator.h
/usr/include/simbody/simmath/SemiExplicitEulerIntegrator.h
/usr/include/simbody/simmath/TimeStepper.h
/usr/include/simbody/simmath/VerletIntegrator.h
/usr/include/simbody/simmath/internal/
/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