x_{1} is a mathematical set/structure/space/object (use appropriate gadri) such that its (set-/structure-/space-/object-theoretic) complement (in space x_{3}) has property x_{2} (ka) in space x_{3}

Can express notions of "cocountability", "cofiniteness", "comeagreness", etc. Do not use for "covector", "cosine", "coordinate", etc.- the prefix "co-" has several meanings in mathematical jargon.