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I've been thinking about the value of writing "of course" in mathematical papers (or its variants such as "obviously" etc.). In particular, my current train of thought is, if something is obvious, then it is obvious that it is obvious (so why include it at all?).

The example that inspired this post is: If d divides a and d divides b, then of course d also divides a+b.

Are there examples in the mathematical literature where the term "of course" is of value?

More precisely, I'm after an example (or a few), ideally by a well-known author, where "of course" or "obviously" or similar actually adds tangible value to a sentence (rather than just implying: (a) it's obvious to me, I'm so smart or (b) I can't actually be bothered working out the details)

PS. I haven't selected community wiki (but I'll keep an eye out if people think it's wanted).

Post Made Community Wiki by Ben Webster
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I've been thinking about the value of writing "of course" in mathematical papers (or it's its variants such as "obviously" etc.). In particular, my current train of thought is, if something is obvious, then it is obvious that it is obvious (so why include it at all?).

The example that inspired this post is: If d divides a and d divides b, then of course d also divides a+b.

Are there examples in the mathematical literature where the term "of course" is of value?

More precisely, I'm after an example (or a few), ideally by a well-known author, where "of course" or "obviously" or similar actually adds tangible value to a sentence (rather than just implying: (a) it's obvious to me, I'm so smart or (b) I can't actually be bothered working out the details)

PS. I haven't selected community wiki (but I'll keep an eye out if people think it's wanted).