Indefinite article before symbols

The choice of "a" versus "an" depends on the pronunciation of the following word. Mathematical writing may use symbols as nouns or adjectives, and their pronunciation is not always clear. Here is a made-up example.

Consider a (⊗,≤,1) triple such that S forms a commutative monoid with (⊗,1) and x ≤ x ⊗ y for any x and y.

In "a (⊗,≤,1) triple", how would one choose the indefinite article to use before the operator?

Yes, you use 'a' or 'an' according to how you pronounce the symbol (or tuple or acronym or letterism). There is no default.

However, things can get a little more complicated than that when writing...
Some say asterisk, some pronounce star
Some spell as a letterism others pronounce as an acronym (a U.P.S. not an UPS)
Some say an before words starting with h and u by rule (an H.I.V. patient not a HIV)
(I don't if the h is pronounced, or I palatalise a word as starting with yu)

In short, say it how you pronounce what follows, and that actually informs people how you pronounce what follows and may help ensure they follow the same convention.

I would probably say "Consider an otimes, less than or equal to, one, triple ...", so I would use "an". You could also say "Consider a tensor, less than or equal to, one, triple", in which case you should use "a".