Dr. Shreyasi Jana Assistant Professor, Dept. of Mathematics Narajole Raj College C3T(SEM II), Unit-I, Intervals Syllabus (UNIT-I, C3T, SEM - II) Intervals, Limit points of a set, Isolated points, Open set, closed set, derived set, Illustrations of Bolzano Weierstrass theorem for sets, compact sets in R, Heine-Borel Theorem. Intervals Definition: An interval is a set of real numbers lying between two numbers called the extremities of the interval. For example, the set of numbers x satisfying 0 ≤ x ≤ 1 is an interval which contains 0, 1 and all numbers in between. Notation for Intervals: The interval of numbers between a and b, including a and b, is often denoted [a, b]. The two numbers are called the endpoints of the interval. Including or excluding endpoints: ∶ (, ) = { ∈ | < < }, , : [, ) = { ∈ | ≤ < }, , : (, ] = { ∈ | < ≤ }, : [,] = { ∈ | ≤ ≤ }.
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Dr. Shreyasi Jana Assistant Professor, Dept. of Mathematics
Narajole Raj College
C3T(SEM II), Unit-I, Intervals
Syllabus (UNIT-I, C3T, SEM - II)
Intervals, Limit points of a set, Isolated points, Open set, closed set, derived set, Illustrations of Bolzano Weierstrass theorem for sets, compact sets in R, Heine-Borel Theorem.
Intervals
Definition: An interval is a set of real numbers lying between two numbers called the extremities of the interval.
For example, the set of numbers x satisfying 0 ≤ x ≤ 1 is an interval which contains 0, 1 and all numbers in between.
Notation for Intervals:
The interval of numbers between a and b, including a and b, is often denoted [a, b]. The
two numbers are called the endpoints of the interval.
Dr. Shreyasi Jana Assistant Professor, Dept. of Mathematics
Narajole Raj College
C3T(SEM II), Unit-I, Intervals
Each interval (a, a), [a, a), and (a, a] represents the empty set, whereas [a, a] denotes the set {a}.
Infinite endpoints
In some contexts, an interval may be defined as a subset of the extended real numbers, the set of all real numbers augmented with −∞ and +∞.
In this interpretation, the notations [−∞, b] , (−∞, b] , [a, +∞] , and [a, +∞) are all meaningful and distinct. In particular, (−∞, +∞) denotes the set of all ordinary real numbers, while [−∞, +∞] denotes the extended reals. (0, +∞) is the set of positive real numbers also written ℝ+.
Dr. Shreyasi Jana Assistant Professor, Dept. of Mathematics
Narajole Raj College
C3T(SEM II), Unit-I, Neighbourhood, Interior Points, Open Sets
Neighbourhood
Definition:Let c ∈ 𝑅. A subset 𝑆 ⊂ 𝑅 is said to be a neighbourhood of c if
there exists an open interval (a,b) such that 𝑐 ∈ (𝑎, 𝑏) ⊂ 𝑆.
Results:
1. The union of two neighbourhoods of c is a neighbourhood of c.
2. The intersection of two neighbourhoods of c is a neighbourhood of
c.
3. The intersection of a finite number of neighbourhoods of c is a
neighbourhood of c.
4. The intersection of an infinite number of neighbourhoods of c is
may not be a neighbourhood of c.
For Example, for every n ∈ 𝑵, (-1/n, 1/n) is a neighbourhood of 0.
⋂ (−𝟏
𝒏,
𝟏
𝒏)∞
𝒏=𝟏 = {0} is not a neighbourhood of 0.
Dr. Shreyasi Jana Assistant Professor, Dept. of Mathematics
Narajole Raj College
C3T(SEM II), Unit-I, Neighbourhood, Interior Points, Open Sets
Interior Point: If S is a subset of a R, then x in S is an interior point of S if there exists a
neighbourhood N(x) of x such that 𝑁(𝑥) ⊂ 𝑆.
Interior: The interior of a set S is the set of all interior points of S. The interior of S is denoted int(S), Int(S) or So. The interior of a set has the following properties.
int(S) is an open subset of S.
int(S) is the union of all open sets contained in S.
int(S) is the largest open set contained in S.
A set S is open if and only if S = int(S).
int(int(S)) = int(S) (idempotence).
If S is a subset of T, then int(S) is a subset of int(T).
If A is an open set, then A is a subset of S if and only if A is a subset of int(S).
Examples:
In any space, the interior of the empty set is the empty set.
If X is the Euclidean space of real numbers, then int([0, 1]) = (0, 1).
The interior of the set of rational numbers is empty.
In any Euclidean space, the interior of any finite set is the empty set.