Can Greek letter names be declined?

Question

There seems to be solid evidence that Latin letter names were indeclinable.

But in Greek, several letters' names do fit into standard declension patterns: sigma, for instance, might actually be a -ma noun from sizō "to hiss". And gamma and digamma look like -ma nouns, even if they historically aren't.

So: do we ever find sigmata or gammata? Or, less obviously, alphai and betai? Or are all letter names indeclinable in Greek?

EDIT: I'd also be interested in cases of omicra and epsila, even though those names are later. But first and foremost I'm curious about usage in the classical period.

Answer 1

LSJ on Σ:

B the name σίγμα ( σῖγμα) was usu. indeclinable, τοῦ σῖγμα Pl. l.c., Cra.402e, 427a, Ath.10.455c, Lyd.Mens.1.21 (v.l. σίγματος) ; τῷ σῖγμα Gal.UP2.14, al.; τῶν σῖγμα Pl.Com.30; τὰ σίγμα τὰ ἐπὶ τῶν ἀσπίδων X.HG4.4.10, cf. Hellad. ap. Phot.Bibl.p.532 B.; later declined, τοῦ σίγματος Eust.1389.15; σίγμασιν Id.905.7.