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MATH-UA 253/MA-UY 3204 - Fall 2022 - Homework #8
发布时间:2022-12-07
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MATH-UA 253/MA-UY 3204 - Fall 2022 - Homework #8
Problem 1. Let G ∈ Rn ×n be symmetric, c ∈ Rn , and d ∈ R. Also, let A ∈ Rm ×n with m < n have full row rank, and let b ∈ Rm . An equality-constrained quadratic program (QP) is a minimization problem of the form:
1. Prove that a set of first-order necessary conditions for optimality for a pair (x∗ , λ ∗ ) are given by: ] [λ(x)
] = [b(−)c] . (2)
2. Let Z ∈ Rn×(n−m) be such that range(Z) = null(A). Show that if the reduced Hessian Z ⊤ GZ is positive definte, then (2) has a unique solution.
3. Show that if the assumptions for the previous part hold, then x∗ is a strict global minimum for (1).
Problem 2. Assume G in (1) is positive definite for this problem. Show that the solution of (2) is given by:
x∗ = G −1A⊤ (AG −1A⊤) AG −1c + G−1 (A⊤ b − c)
λ∗ = (AG −1A⊤) (AG −1c + b) .
(Hint: compute the block LU factorization of the matrix in (2).)
Problem 3. Let x be a guess for x∗ , let p = x∗ − x (the optimal step taking us from x to x∗ ), let h = Ax−b (the degree to which the equality constraints are violated), and let g = c + Gx (the gradient of the cost function evaluated at x). Further, let Z ∈ Rn×(n−m) be a basis matrix for null(A), and let Y ∈ Rn ×m be a basis matrix for null(A)⊥ . Decompose p into its part in the Y and Z bases, i.e. let vY and vZ be such that p = YvY + ZvZ .
1. Show that (2) can be rewritten as:
] [
] = [ ]h(g) . (4)
2. First, show that AYvy = −h.
3. Next, show that Z ⊤ GZvz = −Z⊤ GYvy − Z ⊤g .
4. Assume that Z ⊤ GZ is positive definite. Explain how to solve for p and λ∗ .
Problem 4. The methods outlined for solving an equality-constrained QP in Problems 2 and 3 are two commonly used approaches. Explain the trade-offs between these methods. What assumptions do they require? In terms of the parameters m and n, what is the performance of each method like?