Limit of Composite Function/Counterexample
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Counterexample to Limit of Composite Function is Image of Limit
Let $f$ and $g$ be the real functions defined as:
\(\ds \map g x\) | \(=\) | \(\ds \eta\) | ||||||||||||
\(\ds \map f y\) | \(=\) | \(\ds \begin {cases} y_1 & : y = \eta \\ y_2 & : y \ne \eta \end{cases}\) |
for $\eta, y_1, y_2 \in \R$.
Then:
- $\ds \lim_{y \mathop \to \eta} \map f y = y_2$
and:
- $\ds \lim_{x \mathop \to \xi} \map g x = \eta$
But we have:
- $\forall x \in \R: \map f {\map g x} = y_1$
and so:
- $\ds \lim_{x \mathop \to \xi} \map f {\map g x} = y_1$
Hence it is not true that:
- $\ds \lim_{x \mathop \to \xi} \map f {\map g x} = \lim_{y \mathop \to \eta} \map f y$
Sources
- 1975: W.A. Sutherland: Introduction to Metric and Topological Spaces ... (previous) ... (next): $1$: Review of some real analysis: Exercise $1.5: 15$
- 1977: K.G. Binmore: Mathematical Analysis: A Straightforward Approach ... (previous) ... (next): $\S 8.17$