A function / is continuous at the point a if for any neighborhood
V(f(a)) of its value /(a) at a there is a neighborhood U(a) of a whose
image under the mapping / is contained in V(f(a)).
We now give the expression of this concept in logical symbolism, along with
two other versions of it that are frequently used in analysis.
(/ is continuous at a) := (W(f(a)) 3U(a) (/(17(a)) C V(f(a)))) ,
/e > 0 317(a) Vx G 17(a) (f(x) - f(a) < e) ,
/e>03S>0/xeR(x-a < 6 => f(x)-f(a) < e) .
The equivalence of these statements for real-valued functions follows from
the fact (already noted several times) that any neighborhood of a point contains
a symmetric neighborhood of the point.
For example, if for any ^-neighborhood V£
(f(a)) of f(a) one can choose
a neighborhood U(a) of a such that Vx G U(a) (f(x) — f(a) < e), that
is, f(U(a)) C F£
(/(a)), then for any neighborhood V(/(a)) one can also
choose a corresponding neighborhood of a. Indeed, it suffices first to take
an ^-neighborhood of f(a) with V£
(f(a)) C V(/(a)), and then find U(a)
corresponding to V£
(f(a)). Then f(U(a)) C V£
(f(a)) C V(f(a)).
Thus, if a function is continuous at a in the sense of the second of these
definitions, it is also continuous at a in the sense of the original definition.
The converse is obvious, so that the equivalence of the two statements is
established.
We leave the rest of the verification to the reader.