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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?

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You choose it in exactly the same way: by what follows the indefinite article. How would you say that sentence? – Andrew Leach Mar 12 '13 at 22:13
Effectively, "a" is the "default", so you should use that unless it's certain the next word starts with a vowel sound. Anyway, who's to say an example like yours wouldn't be read out as "Consider a bracket A less-than-or-equal-to B" close-bracket triple blah blah"? – FumbleFingers Mar 12 '13 at 22:15
It depends on how you pronounce it. That's all. Ask a mathematician to pronounce it, and it it starts with a consonant sound, it's a; otherwise it's an. – John Lawler Mar 13 '13 at 3:42
up vote 5 down vote accepted

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.

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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".

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That’s code point U+2297 CIRCLED TIMES. Wouldn’t you just call it “a circled times”? – tchrist Mar 13 '13 at 1:49
@tchrist: I'd call it "oh times"; I think I've heard that, and I don't believe I've heard "circled times". It usually stands for tensor product, and if that's what it means, you could also call it "tensor". It's ⊗ in html. – Peter Shor Mar 13 '13 at 1:50
"Circled times" may be the name for the glyph, but in the math context one would expect math jargon, not typographical jargon. – MετάEd Mar 13 '13 at 3:27
Thanks for the answer about the tensor-product operator. I accepted the other answer because I was really looking for more general guidance. – Heatsink Mar 15 '13 at 15:32

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