, etc., are, like the father of
, terms described by means of a one-many relation (logarithm, cosine, etc.) to a given term (
). The notion of function need not be confined to numbers, or to the uses to which mathematicians have accustomed us; it can be extended to all cases of one-many relations, and "the father of
" is just as legitimately a function of which
is the argument as is "the logarithm of