Properties of the Escape Set and Keep Set
For eachthe escape setand the keep setof the basic quadratic functionhave the following properties

and

is open andis close

and

andare both invariant under

andare both symmetric under rotation byabout 0.

is connected andhas no holes in it
Proof
a) By the backwards form of the triangle inequality(1)
is the positive solution of the quadratic equationWe claim that if then(2) for
Indeed ifthen
as required by (2). (1) and (2) now givefor
Ifthen we can apply this inequality successively toto deduce thatis increasing andas
b) Supposethenasso for someLet Sinceandis a polynomialis continuous atso there existssuch thatimplieshenceIt follows thatimpliesasso thatandis open.
c) The setis not the whole ofbecause it does not include the fixed points of
d)as
as
as
e) Since the set is connected andit is sufficient to show that each pointcan be joined to some point ofby a path inThe proof is by contradiction.
Suppose thatcannot be joined toDefine thensinceis open because ifcan be joined tothen so can points of any open disc inwith centre andis connected because pairs of points incan be joined inviaR is thus a subset ofSincecannot be joined intoandis open we deduce
Now use the maximum principle. Ifthenelse we increase the size ofsofor
Applying the maximum principle to each polynomial functiononwe obtainforandwhich contradicts thathenceis connected.