Theorem 7.11 (Pasting lemma). Let X = A U B, where A, B are open in X. Let f : A → Y and g : B → Y be continuous functions which agree on A nB. Then the function h : AUB → Y defined by h = ƒ on A and h = g on B is continuous.

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Chapter2: Second-order Linear Odes
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Could you explain how to show 7.11 in detail?

Definition. Let X and Y be topological spaces. A function or map f : X → Y is a
continuous function or continuous map if and only if for every open set U in Y,
f-l(U) is open in X.
Definition. Let f : X → Y be a function between topological spaces X and Y, and
let x e X. Then f is continuous at the point x if and only if for every open set
V containing f(x), there is an open set U containing x such that f(U) c V. Thus a
function f : X
→ Y is continuous if and only if it is continuous at each point.
Theorem 7.9. If ƒ : X → Y and g : Y → Z are continuous, then their composition
gof : X → Z is continuous.
We can paste two continuous functions together if the pieces are either both closed
or both open.
AU B, where A, B are closed in X. Let f :
Theorem 7.10 (Pasting lemma). Let X =
A → Y and g : B →
h : AUB → Y defined by h = f on A and h
Y be continuous functions that agree on An B. Then the function
g on B is continuous.
Theorem 7.11 (Pasting lemma). Let X = A U B, where A, B are open in X. Let f :
A → Y and g : B → Y be continuous functions which agree on An B. Then the function
h : AUB-
Y defined by h = ƒ on A and h
= g on B is continuous.
Transcribed Image Text:Definition. Let X and Y be topological spaces. A function or map f : X → Y is a continuous function or continuous map if and only if for every open set U in Y, f-l(U) is open in X. Definition. Let f : X → Y be a function between topological spaces X and Y, and let x e X. Then f is continuous at the point x if and only if for every open set V containing f(x), there is an open set U containing x such that f(U) c V. Thus a function f : X → Y is continuous if and only if it is continuous at each point. Theorem 7.9. If ƒ : X → Y and g : Y → Z are continuous, then their composition gof : X → Z is continuous. We can paste two continuous functions together if the pieces are either both closed or both open. AU B, where A, B are closed in X. Let f : Theorem 7.10 (Pasting lemma). Let X = A → Y and g : B → h : AUB → Y defined by h = f on A and h Y be continuous functions that agree on An B. Then the function g on B is continuous. Theorem 7.11 (Pasting lemma). Let X = A U B, where A, B are open in X. Let f : A → Y and g : B → Y be continuous functions which agree on An B. Then the function h : AUB- Y defined by h = ƒ on A and h = g on B is continuous.
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