where two formulae, f1 and f2, are necessary for adjoining domains but the function is continous at the boundary point, I usually write the description in the form
f(x) = f1(x) for x =< a f(x) = f2(x) for x >= a
The astute reader will note that the domains overlap, but that f1(a) = f2(a), so no semantic ambiguity obtains.
[ This RFC was put into machine readable form for entry ] [ into the online RFC archives by Naoki Matsuhira 5/97 ]