Problem 9: Prove that an upper-triangular matrix is invertible if and only if all of its diagonal entries are nonzero, using the following approach: Let A = (ażj) be an upper- triangular matrix of size n × n. Consider the associated linear map TA F→ Fn. Then : TÃ(еj) = ajj¤j + Σªžj¤i, j=1,2, i

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Problem 9: Prove that an upper-triangular matrix is invertible if and only if all of its
diagonal entries are nonzero, using the following approach: Let A = (aij) be an upper-
triangular matrix of size n × n. Consider the associated linear map TA : F → F.
Then
TÃ(еj) = αjjej + Σαijli, j = 1,2,...,n,
where e₁,e2, ..., en, is the standard basis of Fn. Suppose each diagonal entry aii #
0. Then show, recursively, that e; € im(TÂ), for i 1,2,..., n. Conclude that
=
rank(TA)
n, whence A is invertible. Conversely, suppose that one of the diagonal
entries of A is equal to 0. Show that rank A <n.
-
(Note: By combining problems 4 and 9, we get the theorem that an upper-triangular
matrix is invertible if and only if its determinant is nonzero. Notice that we have not
used the multiplicativity of the determinant in this approach!)
Transcribed Image Text:Problem 9: Prove that an upper-triangular matrix is invertible if and only if all of its diagonal entries are nonzero, using the following approach: Let A = (aij) be an upper- triangular matrix of size n × n. Consider the associated linear map TA : F → F. Then TÃ(еj) = αjjej + Σαijli, j = 1,2,...,n, where e₁,e2, ..., en, is the standard basis of Fn. Suppose each diagonal entry aii # 0. Then show, recursively, that e; € im(TÂ), for i 1,2,..., n. Conclude that = rank(TA) n, whence A is invertible. Conversely, suppose that one of the diagonal entries of A is equal to 0. Show that rank A <n. - (Note: By combining problems 4 and 9, we get the theorem that an upper-triangular matrix is invertible if and only if its determinant is nonzero. Notice that we have not used the multiplicativity of the determinant in this approach!)
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