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In Logic & Mathematics textbooks, the terms 'L-formula', or 'L-term', or 'L-theory' are used. (Though for math, it is reserved to mathematical logic.)

I've witnessed it first-hand, and I'm sure many did also, where authors will alternate between using the articles 'a' & 'an' quite a lot. I assume it's due to the pronunciation: it's odd to say 'a Ell formula' ('an' would be more appropriate in that case) versus 'a L-formula' (do not say 'L'); it raises the question, which is more appropriate?

Examples

Let L be a signature (language), and let the formula X, be an L-formula.

It is the case that a L-theory Y is incomplete if one of its formulae cannot be proven true or false.

All the terms v, w, y belong to the universe of L, and thus an L-term is born.

Note that... The context may be a factor at play, the author may want to remind us that the formula is still an L-formula, but wanting to focus more on proving a point (say, the requirements for something to be a formula), then they might say a L-formula.

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  • No. They cannot ever say "eh ell". It is not grammatical.
    – tchrist ♦
    Commented Aug 15, 2022 at 2:19
  • @tchrist, do you care to clarify?
    – user460756
    Commented Aug 15, 2022 at 4:07

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Whether to use "a" or "an" is determined by the pronunciation. Since I pronounce "L" as ell, I would say "an L-formula". Perhaps others pronounce "L" in other ways; their pronunciation will determine whether they write "a" or "an".

For example, "L" may sometimes be pronounced fifty, but not in this context.

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  • Yes. Oddly enough, "L-formula" begins with a vowel sound (the "e" in "ell"), and so the term should be preceded by "an", not "a".
    – Hot Licks
    Commented Aug 14, 2022 at 23:52
  • @tchrist, no. His answer was beneficial too.
    – user460756
    Commented Aug 15, 2022 at 4:07