Has Επισταμαι been ever used in perfect? I cannot find it in the lexicons.
On doing a search in the way that Joel suggested for the phrase “only in the pres.”, 72 results were returned, slightly less for “only in the pres. and imperf.”. The ἐπίσταμαι, of course, is not among them.
I don’t have access to the TLG here at present, but if you can go online and use a guest account, I think there is a way to choose which forms you can search for from among the extant forms.
The middle of of ιστημι is εστημαι, so it could have been επεστημαι
The middle of of ιστημι is εστημαι,
LSJ reports ἕσταμαι for the perfect passive (and presumably middle), which apparently occurs only in the compound διιστημ:
- Pass., ἵσταμαι: imper. “ἵστασο” Hes.Sc.449, “ἵστω” S.Ph. 893, Ar.Ec.737: impf. ἱστάμην: fut. “στα^θήσομαι” And.3.34, Aeschin. 3.103: more freq. “στήσομαι” Il.20.90, etc.: aor. “ἐστάθην” Od.17.463, etc.; rarely ἔστην, Dor. 3sg. “ἔσστα” SIG56.43 (Argos, v B.C.): > pf. ἕσταμαι （δι-) v.l. in Pl.Ti.81d> , κατεστέαται v.l. in Hdt.1.196.
But what would the perfect of επισταμαι mean? Something like “I am in a state of having arrived at an understanding/having come to know”, but that’s really what the present means: “I am in a state of understanding/knowing”. So there’s really no need for a perfect because the present has a stative meaning.
If the verb ἐπίσταμαι shares its meaning with ἐπιστήμη and ἐπιστήμων. Presumably, then, it is a denominal verb. Denominal means the whole verb is the stem. A perfect might be ἡπιστάμαι.
Does the perfect form of the verb serve a syntactic function, or does each verb only have a meaning to convey of itself in isolation?
Does the perfect form of the verb serve a syntactic function, or does each verb only have a meaning to convey of itself in isolation?
Huh?
The perfect would have a semantic function, but this verb doesn’t have a perfect, and I think that’s for the reason I’ve explained.
ἐπίσταμαι is not a denominative verb. ἐπιστήμη and ἐπιστήμων are deverbative nouns. -μι verbs aren’t derived from nouns by any productive processes. ἐπίσταμαι seems to be a compound of επι + ιστημι, bu the non-psilotic π is puzzling to me. Perhaps it was taken into Attic directly from Ionic. But that’s a guess.
If it needs to be stated openly, everything about the origin of this verb is educated guesses. It goes beyond the normal grammatical rules of morphology for Greek.
I agree with your questioning of the need for a perfect at all.
Yes, I thought “denominative” meant “derived from a noun.” However:
Beekes, R. (2010). A. Lubotsky (Ed.), Etymological Dictionary of Greek (Vol. 1 & 2, p. 445). Leiden; Boston: Brill.
FWIW, ἐπιστάμενος is the present participle of ἐπίσταμαι.
Sorry about that. I picked up the term “denominal” from the Sanskrit grammar I used during my University days. It was the translation of Pāṇini’s नामधातु (nāma-dhātu “nominal verbal root”) and I just assumed the same term was used in Greek circles too. 
-μη is a Greek deverbal noun suffix. Smyth 840a(6) has some examples and in these instances, at least, -μη is added directly to verb stems.
- μα_ (nom. -μη): γνώ-μη knowledge (γι-γνώ-σκω know), φή-μη report, omen (φη-μί say), τι_-μή honour (poet. τί_-ω honour), μνή-μη memory (μι-μνῄ-σκω remind).
ἐπιστήμη is derived from ἐπίσταμαι, not vice versa.
First to harp on a little about the terminology:
I think what you are implying is that these adjectives are used interchangeably.
I say that, because if there was a distinction to be made, the -ative ending, meaning “causing to become”, “having the tendency to form”, “bringing about” is better suited to the description of the suffix, while the simple adjective better describes the resulting noun.
Have I understood you correctly?
I do see your point about the morphology. What I suspect, but haven’t the resources to explore is that the meaning of ἐπιστήμη and ἐπιστήμων was so distinct and specialised, that they invoked (required) that a new verb come into existence to convey just their meanings.
In the light of the etymological dictionary that BH quoted, what I would like to be able to do at the push of a button is to see if there is a correlation between the the meanings that are expressed by these two nouns and the meaning of aorist forms quoted in the entry.