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GNU Emacs Calc 2.02 Manual

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### 3.7.35 Types Tutorial Exercise 3

We can make `inf - inf' be any real number we like, say, a, just by claiming that we added a to the first infinity but not to the second. This is just as true for complex values of a, so `nan` can stand for a complex number. (And, similarly, `uinf` can stand for an infinity that points in any direction in the complex plane, such as `(0, 1) inf').

In fact, we can multiply the first `inf` by two. Surely `2 inf - inf = inf', but also `2 inf - inf = inf - inf = nan'. So `nan` can even stand for infinity. Obviously it's just as easy to make it stand for minus infinity as for plus infinity.

The moral of this story is that "infinity" is a slippery fish indeed, and Calc tries to handle it by having a very simple model for infinities (only the direction counts, not the "size"); but Calc is careful to write `nan` any time this simple model is unable to tell what the true answer is.

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