In: Advanced Math
Problem 17.5. Consider the function χ(0,1) : R → R (this is the characteristic of Definition 14.4). Find: (a) χ(0,1)((0,1)); (b) χ(0,1)((−1,3)); (c) and (in general) χ(0,1)((a,b)), where a,b ∈ R and a < b; prove that the set you found is correct; (d) χ−1 (0,1) ((−2,−1)); (e) χ−1 (0,1) ((0,2)); (f) and (in general) χ−1 (0,1) ((a,b)), where a,b ∈ R and a < b; prove that the set you found is correct,
Definition 14.4 (for Problems 14.6 through 14.9). Let X be a
nonempty set and let
A be a subset of X. The characteristic function or indicator
function of the set A
in X is
χA : X → {0,1} defined by χA(x) = 1 if x ∈ A
0 if x ∈ X\A
From definition,
is given by
.
(a).
. This follows trivially from the above stated definition.
(b).
as for any x in
,
so,
and , combining them we get
.
(c). Case 1.:-
. Then, for any
we have
. So,
.
Case 2.:-
. Here, 0 is in (a,b) and
and
, so
. Hence,
.
Case 3.:-
. Then, similarly as case 2,
.
Case 4:-
. Here, (a,b) is a subset of (0,1) so,
.
Case 5:-
. Then, (a,1) is a subset of (0,1), so
and since
,
. Therefore,
.
Case 6:-
. Here
, so
.
Now observe that,
and
.
(d). As 0 and 1 are both not in (-2,-1), we get
where
denotes the empty set.
(e). As
we get
.
(f). Case 1.-
. Here,
so we get
.
Case 2.-
. Here,
so we get
.
Case 3.-
. Here,
, so we get
.
Case 4.-
. Here,
, so we get
.
Case 5.-
. Here,
so we get
.
Case 6.-
. Here,
so we get
.