I included this footnote in a paper in which I mentioned that the number of partitions of the empty set is 1 (every member of any partition is a non-empty set, and of course every member of the empty set is a non-empty set):


  "Perhaps as a result of studying set theory, I was surprised when I learned that some respectable combinatorialists consider such things as this to be mere convention.  One of them even said a case could be made for setting the number of partitions to 0 when $n=0$.  By stark contrast, Gian-Carlo Rota wrote in \cite{Rota2}, p.~15, that 'the kind of mathematical reasoning that physicists find unbearably pedantic' leads not only to the conclusion that the elementary symmetric function in no variables is 1, but straight from there to the theory of the Euler characteristic, so that 'such reasoning does pay off.'  The only other really sexy example I know is from applied statistics: the non-central chi-square distribution with zero degrees of freedom, unlike its 'central' counterpart, is non-trivial."


The cited paper was: G-C.~Rota, Geometric Probability, Mathematical Intelligencer, 20 (4), 1998, pp. 11--16.  The paper in which my footnote appears is the first one you see here, doi: 10.37236/1027.

Question: What other really gaudy examples are there?

Some remarks:


From one point of view, the whole concept of vacuous truth is silly.  It is a counterintuitive but true proposition that Minneapolis is at a higher latitude than Toronto.  "Ex falso quodlibet" (or whatever the Latin phrase is) and so if you believe Toronto is a more northerly locale than Minneapolis, it will lead you into all sorts of mistakes like $2 + 2 = 5,$ etc.  But that is nonsense.
From another point of view, in its proper mathematical context, it makes perfect sense.
People use examples like propositions about all volcanoes made of pure gold, etc.  That's bad pedagogy and bad in other ways.  What if I ask whether all cell phones in the classroom have been shut off?  If there are no cell phones in the room (that is more realistic than volcanoes made of gold, isn't it??) then the correct answer is "yes".  That's a good example, showing, if only in a small way, the utility of the concept when used properly.
I don't think it's mere convention that the number of partitions of the empty set is 1; it follows logically from some basic things in logic.  Those don't make sense in some contexts (see "Minneapolis", "Toronto", etc., above) but in fact the only truth value that can be assigned to $\text{“}F\Longrightarrow F\text{''}$ or $\text{“}F\Longrightarrow T\text{''}$ that makes it possible to fill in the truth table without knowing the content of the false proposition (and satisfies the other desiderata?) is $T.$  That's a fact whose truth doesn't depend on conventions.


How many open covers does the empty topological space have?  Not one, not none, but two: the empty cover $\varnothing$, since its union is $\bigcup\varnothing=\varnothing$, and the cover $\{\varnothing\}$, since its union is also $\bigcup\{\varnothing\} =\varnothing$.
This comes up when using the Grothendieck plus-construction to sheafify a presheaf.  Apply the construction to the (nonseparated) presheaf $P:\mathcal{O}(X)^\mathrm{op}\to \mathrm{Set}$ sending every open set to the set $A$, with $|A|\geq 2$.  Then the presheaf $P^+:\mathcal{O}(X)^\mathrm{op}\to\mathrm{Set}$ agrees with $P$ on every open set except $\varnothing\subseteq X$, where $P^+(\varnothing)$ is now a one-element set $\{*\}.$  This is because the matching families for the cover $\{\varnothing\}$ of $\varnothing$ (of which there is one for each $a\in A$) are all set equal to the unique matching family for the refining cover $\varnothing\subseteq\{\varnothing\}$ of $\varnothing$.
This elementary example comes from "Sheaves in Geometry and Logic", by Moerdijk and MacLane.
