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Exercise 288Topology › Topology of Mn(ℝ) › Applications and the case n = 2L2Added 6 Nov 2026

The inverse does not extend continuously across singular matrices

#inversion#unboundedness#continuity#extension

Statement

Show that there is no continuous map F:Mn(R)→Mn(R)F:M_n(\mathbb{R})\to M_n(\mathbb{R}) with F(A)=A−1F(A)=A^{-1} for every invertible AA.

Hint

Look at the matrices εI\varepsilon I as ε→0+\varepsilon\to0^+.

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Solution

Idea. A continuous map is bounded on a neighbourhood of any point, but the inverse blows up as we approach a singular matrix.

Step 1: suppose FF exists. Then FF is continuous at 00, so there are δ>0\delta>0 and M<∞M<\infty with ∥F(A)∥≤M\|F(A)\|\le M whenever ∥A∥<δ\|A\|<\delta (norm of your choice).

Step 2: test matrices. For 0<ε<δ/n0<\varepsilon<\delta/\sqrt n the matrix εI\varepsilon I is invertible with ∥εI∥F=εn<δ\|\varepsilon I\|_F=\varepsilon\sqrt n<\delta, and
∥F(εI)∥F=∥ε−1I∥F=nε.\|F(\varepsilon I)\|_F=\|\varepsilon^{-1}I\|_F=\frac{\sqrt n}{\varepsilon}.
This tends to +∞+\infty as ε→0+\varepsilon\to0^+, contradicting ∥F(εI)∥≤M\|F(\varepsilon I)\|\le M.

Remark. The same argument works at any singular matrix A0A_0. Choose x≠0x\ne0 with A0x=0A_0x=0. Then (A0+εI)x=εx(A_0+\varepsilon I)x=\varepsilon x, so for small ε≠0\varepsilon\ne0 (where A0+εIA_0+\varepsilon I is invertible) (A0+εI)−1x=x/ε(A_0+\varepsilon I)^{-1}x=x/\varepsilon, hence ∥(A0+εI)−1∥op≥1/ε→∞\|(A_0+\varepsilon I)^{-1}\|_{op}\ge1/\varepsilon\to\infty. So the inverse cannot be extended continuously to any point of the singular set: GLn(R)GL_n(\mathbb{R}) is the maximal domain on which inversion is continuous (in the sense of extension to a larger set of limit points).

Lesson. Unboundedness near a point kills any continuous extension.