A simple (though somewhat pedantic) question: when using big O notation in a sentence, should it be preceded with "a" or "an"? Example:

The extra loop results in a/an $\mathcal{O}(N)$ increase in cost.

The ubiquitous Cormen textbook notes on page 47:

For a given function $g(n)$, we denote by $\mathcal{O}(g(n))$ (pronounced “big-oh of g of n” or sometimes just “oh of g of n”)

which would seem to indicate "a" should precede something pronounced "big-oh". Later on the same page, however, the textbook uses the phrase "an $\mathcal{O}(n^2)$ upper bound." Which is used more in the literature? It seems like a triviality, but as someone working on the edge of theoretical computer science from a primary focus on another field, I'd like not to sound like an outsider.

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    $\begingroup$ I think the common way to pronounce the offending sentence would have just “oh en increase” with no “big”, so an “an” is appropriate. $\endgroup$ Commented Oct 8, 2014 at 16:54
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    $\begingroup$ Speaking of pedantry, the proper way to typeset the big-oh notation, as it has been used for over a century, is $O(\dots)$. (I observe that the Cormen textbook agrees, I don’t know why you changed it in the quote.) The caligraphic $\mathcal O$ just displays a lack of taste. $\endgroup$ Commented Oct 8, 2014 at 17:11
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    $\begingroup$ (Tell me to shut up if I’m annoying.) While we are on the subject: on the other hand, a calligraphic $\mathcal P$ is an appropriate power-set notation. A depressingly large number of people abuse for that purpose the symbol for the Weierstraß elliptic function $\wp(z;\tau)$. $\endgroup$ Commented Oct 8, 2014 at 19:34
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    $\begingroup$ I may be the only one, but when I am wondering about similar issues I mentally expand "$O(N)$ increase" to "order of $N$ increase". $\endgroup$ Commented Oct 8, 2014 at 20:33
  • $\begingroup$ "...increases the running time by a factor of O(n)." (And pace Cormen, I've always pronounced it "order en".) $\endgroup$
    – Jeffε
    Commented Oct 12, 2014 at 16:29

1 Answer 1


Evidence for "an":

Of course, these examples alone merely suggest that "an" is acceptable. This list does not show that "an" is necessarily standard or preferred nor does it show that "a" is unacceptable.


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