/usr/include/Rivet/Math/ is in librivet-dev 1.8.3-1.1.
This file is owned by root:root, with mode 0o755.
..
/usr/include/Rivet/Math/Constants.hh
/usr/include/Rivet/Math/LorentzTrans.hh
/usr/include/Rivet/Math/Math.hh
/usr/include/Rivet/Math/MathHeader.hh
/usr/include/Rivet/Math/MathUtils.hh
/usr/include/Rivet/Math/Matrices.hh
/usr/include/Rivet/Math/Matrix3.hh
/usr/include/Rivet/Math/MatrixDiag.hh
/usr/include/Rivet/Math/MatrixN.hh
/usr/include/Rivet/Math/Units.hh
/usr/include/Rivet/Math/Vector3.hh
/usr/include/Rivet/Math/Vector4.hh
/usr/include/Rivet/Math/VectorN.hh
/usr/include/Rivet/Math/Vectors.hh
/usr/include/Rivet/Math/eigen/
/usr/include/Rivet/Math/eigen/linearsolver.h
/usr/include/Rivet/Math/eigen/linearsolverbase.h
/usr/include/Rivet/Math/eigen/ludecomposition.h
/usr/include/Rivet/Math/eigen/ludecompositionbase.h
/usr/include/Rivet/Math/eigen/matrix.h
/usr/include/Rivet/Math/eigen/matrixbase.h
/usr/include/Rivet/Math/eigen/projective.h
/usr/include/Rivet/Math/eigen/regression.h
/usr/include/Rivet/Math/eigen/regressioninternal.h
/usr/include/Rivet/Math/eigen/util.h
/usr/include/Rivet/Math/eigen/vector.h
/usr/include/Rivet/Math/eigen/vectorbase.h