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Multivariable Advanced Calculus

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58 BASIC LINEAR ALGEBRA3.8.5 Orthonormal BasesNot all bases for an inner product space H are created equal.The best bases areorthonormal.Definition 3.8.11 Suppose {v 1 , · · · , v k } is a set of vectors in an inner productspace H. It is an orthonormal set ifv i · v j = δ ij ={ 1 if i = j0 if i ≠ jEvery orthonormal set of vectors is automatically linearly independent.Proposition 3.8.12 Suppose {v 1 , · · · , v k } is an orthonormal set of vectors. Thenit is linearly independent.Proof: Suppose ∑ ki=1 c iv i = 0. Then taking dot products with v j ,0 = 0 · v j = ∑ ic i v i · v j = ∑ ic i δ ij = c j .Since j is arbitrary, this shows the set is linearly independent as claimed.It turns out that if X is any subspace of H, then there exists an orthonormal basisfor X.Lemma 3.8.13 Let X be a subspace of dimension n whose basis is {x 1 , · · · , x n } .Then there exists an orthonormal basis for X, {u 1 , · · · , u n } which has the property thatfor each k ≤ n, span(x 1 , · · · , x k ) = span (u 1 , · · · , u k ) .Proof: Let {x 1 , · · · , x n } be a basis for X. Let u 1 ≡ x 1 / |x 1 | . Thus for k = 1,span (u 1 ) = span (x 1 ) and {u 1 } is an orthonormal set. Now suppose for some k

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