In a mathematical context,which was published in 1956,I noticed a weird construction;in which "made" was used in a dubious way.
Here's The Sentence
When increasing and decreasing quantities are made the subject of mathematical investigation.
Firstly I consider the term "made" is functioning as an adjective but I found the negative result.It isn't used in the way "made" is used as an adjectival form i.e man-made,Japanese-made etc.
secondly,I assumed it as a verb and the sentence as a passive one ,which implies the presence of some hidden doer, I assume the doer as "X" so the new sentence is "when increasing and decreasing quantities are made by X the subject of mathematical investigation", which perceived me as follow "when increasing and decreasing quantities becomes the subject of investigation."
what I concluded from above is that the term "Made" is the synonym of "Become". Since I am not a native English speaker,I may be wrong.