I've decided that zero and negative integers could be useful to me. All
negative integers are divisible by positive integers (if, on occasions,
only
themselves and -1), aren't they? Should I, then, stick with |N, but define
it as 'all integers', or shall I use |Z, and then make all my functions a
function from |Z to |Z? It looks as though I'll need a definition of |N in
any case, for when I'm saying such things as b \in |N.
With thanks.
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