Show that if det A1:k,1:k > 0 for all k = 1, 2,... , m, then A is positive definite. Hint: Show A has a Choleski decomposition.

Linear Algebra: A Modern Introduction
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ISBN:9781285463247
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Chapter4: Eigenvalues And Eigenvectors
Section4.3: Eigenvalues And Eigenvectors Of N X N Matrices
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2. Let A e Cmxm be Hermitian.
(a) Show that if det A1:k.1:k > 0 for all k = 1, 2, ... , m, then A is positive definite.
Hint: Show A has a Choleski decomposition.
(b) Suppose A is positive definite. Show that the largest elements in magnitude of A appear
on the diagonal and are positive real numbers.
[
akk
akl
Hint: All principle minors of an HPD matrix are positive. Thus, 0 < det
a ek
all
for all k, l e {1, 2, ..., m} with k + l.
(c) Suppose A is positive definite. Show that MAM* is Hermitian positive definite if M is
nonsingular.
Note: This property can be used to show that the Schur complement of the matrix A, as
defined in the previous homework, is HPD.
Transcribed Image Text:2. Let A e Cmxm be Hermitian. (a) Show that if det A1:k.1:k > 0 for all k = 1, 2, ... , m, then A is positive definite. Hint: Show A has a Choleski decomposition. (b) Suppose A is positive definite. Show that the largest elements in magnitude of A appear on the diagonal and are positive real numbers. [ akk akl Hint: All principle minors of an HPD matrix are positive. Thus, 0 < det a ek all for all k, l e {1, 2, ..., m} with k + l. (c) Suppose A is positive definite. Show that MAM* is Hermitian positive definite if M is nonsingular. Note: This property can be used to show that the Schur complement of the matrix A, as defined in the previous homework, is HPD.
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