In the example below, to which word class do "so" and "such" belong?
A function is measurable if and only if so / such are its positive and negative parts.
Note that "so"/"such" are not intensifiers, but are in some pronominal role. The intended meaning is
A function is measurable if and only if its positive and negative parts are measurable.
One can argue that "such" refers to the adjective "measurable", rather than determines or qualifies any noun. Therefore, "such" must be a pronoun and not a determiner. In other languages pronouns can refer to nouns, adjectives and some other parts of speech. Can English pronouns refer to adjectives? Would it be more appropriate to class them as pronominal adjectives?
Another point of view comes from the syntactic analysis of the first clause "A function is measurable", in which "is measurable" is a verb phrase. Since "measurable" does not modify any noun directly, it is essentially in a role of an adverb. That is, "measurable" is a mode of "being", just like "quickly" is a mode of "doing". Consequently, "such"/"so" must be pronominal adverbs, because they refer to an adverb and modify the verb "are".
Are both points of view valid? Are there any mistakes in the analyses?
Background information on the sentence in question.
A quick internet search shows that this pattern is widespread in mathematical literature. I have a feeling that "so" is more popular, and "such" is often used by European foreign speakers.
The usage of "so" and "such" in a non-intensifier role has not been much discussed here. I found only one related question "so" or "such" to replace a preceding adjective, which discuss a slightly different setting. Therein, the accepted answer insists that "such" normally refers to a noun, but I believe that "such" may equally refer to an adjective (i.e. to some quality of a noun rather than a specific noun), similarly to "what". Am I mistaken?