/usr/share/xml/schema/w3c/mathml/dtd/ is in w3-dtd-mathml 2.0.0.0-5.
This file is owned by root:root, with mode 0o755.
..
/usr/share/xml/schema/w3c/mathml/dtd/catalog
/usr/share/xml/schema/w3c/mathml/dtd/catalog.xml
/usr/share/xml/schema/w3c/mathml/dtd/isoamsa.ent
/usr/share/xml/schema/w3c/mathml/dtd/isoamsb.ent
/usr/share/xml/schema/w3c/mathml/dtd/isoamsc.ent
/usr/share/xml/schema/w3c/mathml/dtd/isoamsn.ent
/usr/share/xml/schema/w3c/mathml/dtd/isoamso.ent
/usr/share/xml/schema/w3c/mathml/dtd/isoamsr.ent
/usr/share/xml/schema/w3c/mathml/dtd/isobox.ent
/usr/share/xml/schema/w3c/mathml/dtd/isocyr1.ent
/usr/share/xml/schema/w3c/mathml/dtd/isocyr2.ent
/usr/share/xml/schema/w3c/mathml/dtd/isodia.ent
/usr/share/xml/schema/w3c/mathml/dtd/isogrk3.ent
/usr/share/xml/schema/w3c/mathml/dtd/isolat1.ent
/usr/share/xml/schema/w3c/mathml/dtd/isolat2.ent
/usr/share/xml/schema/w3c/mathml/dtd/isomfrk.ent
/usr/share/xml/schema/w3c/mathml/dtd/isomopf.ent
/usr/share/xml/schema/w3c/mathml/dtd/isomscr.ent
/usr/share/xml/schema/w3c/mathml/dtd/isonum.ent
/usr/share/xml/schema/w3c/mathml/dtd/isopub.ent
/usr/share/xml/schema/w3c/mathml/dtd/isotech.ent
/usr/share/xml/schema/w3c/mathml/dtd/mathml2-qname-1.mod
/usr/share/xml/schema/w3c/mathml/dtd/mathml2.dtd
/usr/share/xml/schema/w3c/mathml/dtd/mmlalias.ent
/usr/share/xml/schema/w3c/mathml/dtd/mmlextra.ent
/usr/share/xml/schema/w3c/mathml/dtd/xhtml-math11-f.dtd