# Spivak's calculus on manifolds chapter 1

1. Jul 4, 2008

### Adeimantus

1. The problem statement, all variables and given/known data
1. Problem 1-8(b)

If there is a basis x1,..., xn of Rn and numbers a1,..., an such that Txi = aixi (where T is a linear operator), prove that T is angle-preserving if and only if all the |ai| are equal.

2. Relevant equations
First a couple of definitions...

Definition (i): If x,y are nonzero vectors in Rn, the angle between them is given by

angle(x,y) = arccos {<x,y>/ ( |x | |y | )}

where < , > denotes the standard inner product and | | the standard norm.

Definition (ii): A linear transformation T is angle-preserving if T is 1-1 and for x,y != 0 we have that angle(Tx, Ty) = angle(x,y).

3. The attempt at a solution
Problem is, I think I see a simple counterexample to the sufficiency part of the proof. Let x1, x2 be a basis for R2 where

x1 = e1 (i.e. standard unit basis vector) and
x2 = e1 + e2

Then suppose Tx1 = x1 and Tx2 = -x2. Now consider the angle between x1 and x1+x2:

angle(x1, x1+x2) = angle(e1, 2e1+e2) ~ 25 degrees, but

angle(Tx1,T(x1+x2)) = angle(x1, x1-x2) = angle(e1, -e2) = 90 degrees.

So even though the absolute values of a1 = 1 and a2 = -1 are equal, it seems T does not preserve angles. What am I misunderstanding about this problem? I am assuming the author means angle-preserving with respect to the standard inner product in which <e1, e2> = 0.

Last edited: Jul 4, 2008
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