In speaking or writing about mathematics, it is important to keep in mind that the indefinite article “a” becomes “an” when it precedes a vowel. Thus one says “an island,” and not “a island.” While this may perhaps already be second nature to you in nonmathematical discourse, you have to be particularly alert in your mathematical writing. The reason for this is that the choice between “a” and “an” depends not on the way that something is written, but on how it is pronounced. Fans of popular music may recall the appearance of Country Joe McDonald at the legendary Woodstock Festival: “Gimme an F! Gimme a U!” Which is completely counterintutive. Even though F is a consonant, the name of the letter is pronounced “eff,” which begins with a vowel. In the case of U, the situation is reversed. The pronunciation is “you,” which begins with the consonant Y, and that is why one says “a U.” And of course in mathematical discourse, individual letters are encountered in quantity. Thus one says (and writes) “an M-ideal,” “an NP-hard problem,” “an Hp-space,” “a U-test,” “a Y-flow” (and keep in mind that in English, the letter Y is pronounced “wye,” not “vie,” as it might be in some other languages).
And then there is the question of words that begin with the letter H. In “hour,” the H is silent, whereas in “history,” it is pronounced. Therefore, one writes “an hour,” but “a history lesson.” There are some people who say or write “an historic occasion,” “an horrific event,” or “an hotel.” This, however, reflects a pronunciation that was common in the eighteenth and nineteenth centuries, but has since died out. Therefore, since the H is in fact pronounced in those three words, one should write, and say, “a historic occasion,” “a horrific event,” and “a hotel.” And there is one more bit of ambiguity: the British pronounce the H in “herb,” while the Americans do not.
Finally, one has to know how the Greek letters are pronounced, since they appear frequently in mathematical writing. One of course says and writes “an \(\alpha\)” (an alpha) “an \(\epsilon\)” (an epsilon), “ “an \(\omega\)” (an omega), but “a \(\beta\)” (a beta), “a \(\gamma\)” (a gamma), and so on. As in the case of “herb” mentioned above, there are pronunciation differences regarding the names of the Greek letters between the British and the Americans. For the letters beta and zeta, the British say beeta and zeeta, to rhyme with cheetah, while the Americans say bayta and zayta. The Americans say epsuhlon, with the accent on the first syllable, while the British say epsylon (to rhyme with nylon), with the accent on the second syllable.