# Lagrange Duality. Daniel P. Palomar. Hong Kong University of Science and Technology (HKUST)

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1 Lagrange Duality Daniel P. Palomar Hong Kong University of Science and Technology (HKUST) ELEC Convex Optimization Fall , HKUST, Hong Kong

2 Outline of Lecture Lagrangian Dual function Dual problem Weak and strong duality KKT conditions Summary (Acknowledgement to Stephen Boyd for material for this lecture.) Daniel P. Palomar 1

3 Lagrangian Consider an optimization problem in standard form (not necessarily convex) minimize x f 0 (x) subject to f i (x) 0 i = 1,..., m h i (x) = 0 i = 1,..., p with variable x R n, domain D, and optimal value p. The Lagrangian is a function L : R n R m R p R, with dom L = D R m R p, defined as m p L (x, λ, ν) = f 0 (x) + λ i f i (x) + ν i h i (x) i=1 where λ i is the Lagrange multiplier associated with f i (x) 0 and ν i is the Lagrange multiplier associated with h i (x) = 0. i=1 Daniel P. Palomar 2

4 Lagrange Dual Function The Lagrange dual function is defined as the infimum of the Lagrangian over x: g : R m R p R, g (λ, ν) = inf L (x, λ, ν) x D ( = inf x D f 0 (x) + m λ i f i (x) + i=1 ) p ν i h i (x) Observe that: the infimum is unconstrained (as opposed to the original constrained minimization problem) g is concave regardless of original problem (infimum of affine functions) g can be for some λ, ν i=1 Daniel P. Palomar 3

5 Lower bound property: if λ 0, then g (λ, ν) p. Proof. Suppose x is feasible and λ 0. Then, f 0 ( x) L ( x, λ, ν) inf L (x, λ, ν) = g (λ, ν). x D Now choose minimizer of f 0 ( x) over all feasible x to get p g (λ, ν). We could try to find the best lower bound by maximizing g (λ, ν). This is in fact the dual problem. Daniel P. Palomar 4

6 Dual Problem The Lagrange dual problem is defined as maximize λ,ν g (λ, ν) subject to λ 0. This problem finds the best lower bound on p obtained from the dual function. It is a convex optimization (maximization of a concave function and linear constraints). The optimal value is denoted d. λ, ν are dual feasible if λ 0 and (λ, ν) dom g (the latter implicit constraints can be made explicit in problem formulation). Daniel P. Palomar 5

7 Example: Least-Norm Solution of Linear Equations Consider the problem The Lagrangian is minimize x x T x subject to Ax = b. L (x, ν) = x T x + ν T (Ax b). To find the dual function, we need to solve an unconstrained minimization of the Lagrangian. We set the gradient equal to zero x L (x, ν) = 2x + A T ν = 0 = x = (1/2) A T ν Daniel P. Palomar 6

8 and we plug the solution in L to obtain g: g (ν) = L ( (1/2) A T ν, ν ) = 1 4 νt AA T ν b T ν The function g is, as expected, a concave function of ν. From the lower bound property, we have p 1 4 νt AA T ν b T ν for all ν. The dual problem is the QP maximize ν 1 4 νt AA T ν b T ν. Daniel P. Palomar 7

9 Consider the problem Example: Standard Form LP minimize x c T x subject to Ax = b, x 0. The Lagrangian is L (x, λ, ν) = c T x + ν T (Ax b) λ T x = ( c + A T ν λ ) T x b T ν. L is a linear function of x and it is unbounded if the term multiplying x is nonzero. Daniel P. Palomar 8

10 Hence, the dual function is g (λ, ν) = inf x L (x, λ, ν) = { b T ν c + A T ν λ = 0 otherwise. The function g is a concave function of (λ, ν) as it is linear on an affine domain. From the lower bound property, we have The dual problem is the LP p b T ν if c + A T ν 0. maximize ν b T ν subject to c + A T ν 0. Daniel P. Palomar 9

11 Consider the problem Example: Two-Way Partitioning minimize x x T W x subject to x 2 i = 1, i = 1,..., n. It is a nonconvex problem (quadratic equality constraints). feasible set contains 2 n discrete points. The Lagrangian is n L (x, ν) = x T ( W x + ν i x 2 i 1 ) i=1 = x T (W + diag (ν)) x 1 T ν. The L is a quadratic function of x and it is unbounded if the matrix W + diag (ν) has a negative eigenvalue. Daniel P. Palomar 10

12 Hence, the dual function is g (ν) = inf x L (x, ν) = { 1 T ν W + diag (ν) 0 otherwise. From the lower bound property, we have p 1 T ν if W + diag (ν) 0. As an example, if we choose ν = λ min (W ) 1, we get the bound The dual problem is the SDP p nλ min (W ). maximize ν 1 T ν subject to W + diag (ν) 0. Daniel P. Palomar 11

13 Weak and Strong Duality From the lower bound property, we know that g (λ, ν) p for feasible (λ, ν). In particular, for a (λ, ν) that solves the dual problem. Hence, weak duality always holds (even for nonconvex problems): d p. The difference p d is called duality gap. Solving the dual problem may be used to find nontrivial lower bounds for difficult problems. Daniel P. Palomar 12

14 Even more interesting is when equality is achieved in weak duality. This is called strong duality: d = p. Strong duality means that the duality gap is zero. Strong duality: is very desirable (we can solve a difficult problem by solving the dual) does not hold in general usually holds for convex problems conditions that guarantee strong duality in convex problems are called constraint qualifications. Daniel P. Palomar 13

15 Slater s Constraint Qualification Slater s constraint qualification is a very simple condition that is satisfied in most cases and ensures strong duality for convex problems. Strong duality holds for a convex problem minimize x f 0 (x) subject to f i (x) 0 i = 1,..., m Ax = b if it is strictly feasible, i.e., x int D : f i (x) < 0 i = 1,..., m, Ax = b. Daniel P. Palomar 14

16 It can be relaxed by using relint D (interior relative to affine hull) instead of int D; linear inequalities do not need to hold with strict inequality,... There exist many other types of constraint qualifications. Daniel P. Palomar 15

17 Consider the problem The dual problem is Example: Inequality Form LP minimize x c T x subject to Ax b. maximize λ b T λ subject to A T λ + c = 0, λ 0. From Slater s condition: p = d if A x < b for some x. In this case, in fact, p = d infeasible. except when primal and dual are Daniel P. Palomar 16

18 Example: Convex QP Consider the problem (assume P 0) The dual problem is minimize x x T P x subject to Ax b. maximize λ (1/4) λ T AP 1 A T λ b T λ subject to λ 0. From Slater s condition: p = d if A x < b for some x. In this case, in fact, p = d always. Daniel P. Palomar 17

19 Example: Nonconvex QP Consider the problem minimize x x T Ax + 2b T x subject to x T x 1 which is nonconvex in general as A 0. The dual problem is maximize λ b T (A + λi) # b λ subject to A + λi 0 b R (A + λi) Daniel P. Palomar 18

20 which can be rewritten as maximize t,λ subject to t λ [ A + λi b b T t ] 0. In this case, strong duality holds even though the original problem is nonconvex (not trivial). Daniel P. Palomar 19

21 Complementary Slackness Assume strong duality holds, x is primal optimal and (λ, ν ) is dual optimal. Then, ( ) m p f 0 (x ) = g (λ, ν ) = inf f 0 (x) + λ i f i (x) + νi h i (x) x f 0 (x ) + f 0 (x ) i=1 m λ i f i (x ) + i=1 i=1 p νi h i (x ) Hence, the two inequalities must hold with equality. Implications: x minimizes L (x, λ, ν ) λ i f i (x ) = 0 for i = 1,..., m; this is called complementary slackness: λ i > 0 = f i (x ) = 0, f i (x ) < 0 = λ i = 0. i=1 Daniel P. Palomar 20

22 Karush-Kuhn-Tucker (KKT) Conditions KKT conditions (for differentiable f i, h i ): 1. primal feasibility: f i (x) 0, i = 1,..., m, h i (x) = 0, i = 1,..., p 2. dual feasibility: λ 0 3. complementary slackness: λ i f i (x ) = 0 for i = 1,..., m 4. zero gradient of Lagrangian with respect to x: f 0 (x) + m λ i f i (x) + p ν i h i (x) = 0 i=1 i=1 Daniel P. Palomar 21

23 We already known that if strong duality holds and x, λ, ν are optimal, then they must satisfy the KKT conditions. What about the opposite statement? If x, λ, ν satisfy the KKT conditions for a convex problem, then they are optimal. Proof. From complementary slackness, f 0 (x) = L (x, λ, ν) and, from 4th KKT condition and convexity, g (λ, ν) = L (x, λ, ν). Hence, f 0 (x) = g (λ, ν). Theorem. If a problem is convex and Slater s condition is satisfied, then x is optimal if and only if there exists λ, ν that satisfy the KKT conditions. Daniel P. Palomar 22

24 Example: Waterfilling Solution Consider the maximization of the mutual information in a MIMO channel under Gaussian noise: maximize Q subject to log det ( R n + HQH ) Tr (Q) P Q 0. This problem is convex: the logdet function is concave, the trace constraint is just a linear constraint, and the positive semidefiniteness constraint is an LMI. Hence, we can use a general-purpose method such as an interiorpoint method to solve it in polynomial time. Daniel P. Palomar 23

25 However, this problem admits a closed-form solution as can be derived from the KKT conditions. The Lagrangian is L (Q; µ, Ψ) = log det ( R n + HQH ) +µ (Tr (Q) P ) Tr (ΨQ). The gradient of the Lagrangian is Q L = H ( R n + HQH ) 1 H + µi Ψ. Daniel P. Palomar 24

26 The KKT conditions are Tr (Q) P, Q 0 µ 0, Ψ 0 H ( R n + HQH ) 1 H + Ψ = µi µ (Tr (Q) P ) = 0, ΨQ = 0. Can we find a Q that satisfies the KKT conditions (together with some dual variables)? Daniel P. Palomar 25

27 First, let s simplify the KKT conditions by defining the so-called whitened channel: H = R 1/2 n H. Then, the third KKT condition becomes: H ( I + HQ H ) 1 H + Ψ = µi. To simplify even further, let s write the SVD of the channel matrix as H = UΣV (denote the eigenvalues σ i ), obtaining: Σ ( I + Σ QΣ ) 1 Σ + Ψ = µi. where Q = V QV and Ψ = V ΨV. Daniel P. Palomar 26

28 The KKT conditions are: Tr( Q) P, Q 0 µ 0, Ψ 0 ( Σ I + Σ QΣ ) 1 Σ + Ψ = µi ( µ Tr( Q) ) P = 0, Ψ Q = 0. At this point, we can make a guess: perhaps the optimal Q and Ψ are diagonal? Let s try... Daniel P. Palomar 27

29 Define Q = diag (p) (p is the power allocation) and Ψ = diag (ψ). The KKT conditions become: i p i P, p i 0 µ 0, ψ i 0 µ ( i σ 2 i 1 + σi 2p i ) + ψ i = µ p i P = 0, ψ i p i = 0. Let s now look into detail at the KKT conditions. Daniel P. Palomar 28

30 First of all, observe that µ > 0, otherwise we would have ψ i = 0 which cannot be satisfied. σ 2 i 1+σ 2 i p i + Let s distinguish two cases in the power allocation: if p i > 0, then ψ i = 0 = σ2 i 1+σ 2 i p i = µ = p i = µ 1 1/σ 2 i (also note that µ = σ2 i < 1+σ i 2p σ2 i ) i if p i = 0, then σi 2 + ψ i = µ (note that µ = σi 2 + ψ i σi 2. Equivalently, if σ 2 i > µ, then p i = µ 1 1/σ 2 i if σ 2 i µ, then p i = 0. Daniel P. Palomar 29

31 More compactly, we can write the well-known waterfilling solution: p i = ( µ 1 1/σ 2 i ) + where µ 1 is called water-level and is chosen to satisfy i p i = P (so that all the KKT conditions are satisfied). Therefore, the optimal solution is given by where Q = Vdiag (p) V the optimal transmit directions are matched to the channel matrix the optimal power allocation is the waterfilling. Daniel P. Palomar 30

32 Perturbation and Sensitivity Analysis Recall the original (unperturbed) optimization problem and its dual: minimize x f 0 (x) subject to f i (x) 0 i h i (x) = 0 i maximize λ,ν g (λ, ν) subject to λ 0 Define the perturbed problem and dual as minimize x f 0 (x) subject to f i (x) u i i h i (x) = v i i maximize λ,ν g (λ, ν) u T λ v T ν subject to λ 0 x is primal variable and u, v are parameters Define p (u, v) as the optimal value as a function of u, v. Daniel P. Palomar 31

33 Global sensitivity: Suppose strong duality holds for unperturbed problem and λ, ν are dual optimal for unperturbed problem. Then, from weak duality: Interpretation: p (u, v) g (λ, ν ) u T λ v T ν = p (0, 0) u T λ v T ν if λ i large: p increases a lot if we tighten constraint i (u i < 0) if λ i small: p does not decrease much if we loosen constraint i (u i > 0) if ν i large and positive: p increases a lot if we take v i < 0 if ν i large and negative: p increases a lot if we take v i > 0 etc. Daniel P. Palomar 32

34 Local sensitivity: Suppose strong duality holds for unperturbed problem, λ, ν are dual optimal for unperturbed problem, and p (u, v) is differentiable at (0, 0). Then, p (0, 0) u i = λ i, p (0, 0) v i = ν i Proof. (for λ i )From the global sensitivity result, we have p (0, 0) u i = lim ɛ 0 p (te i, 0) p (0, 0) t lim ɛ 0 tλ i t = λ i p (0, 0) u i Hence, the equality. = lim ɛ 0 p (te i, 0) p (0, 0) t lim ɛ 0 tλ i t = λ i. Daniel P. Palomar 33

35 Duality and Problem Reformulations Equivalent formulations of a problem can lead to very different duals. Reformulating the primal problem can be useful when the dual is difficult to derive or uninteresting. Common tricks: introduce new variables and equality constraints make explicit constraints implicit or vice-versa transform objective or constraint functions (e.g., replace f 0 (x) by φ (f 0 (x)) with φ convex and increasing). Daniel P. Palomar 34

36 Example: Introducing New Variables Consider the problem minimize x Ax b 2. We can rewrite it as minimize x,y y 2 subject to y = Ax b. We can then derive the dual problem: maximize ν b T ν subject to A T ν = 0, ν 2 1. Daniel P. Palomar 35

37 Example: Implicit Constraints Consider the following LP with box constrains: minimize x subject to c T x Ax = b 1 x 1 The dual problem is maximize ν,λ 1,λ 2 b T ν 1 T λ 1 1 T λ 2 subject to c + A T ν + λ 1 λ 2 = 0 λ 1 0, λ 2 0, which does not give much insight. Daniel P. Palomar 36

38 If, instead, we rewrite the primal problem as minimize x subject to f 0 (x) = Ax = b { c T x 1 x 1 otherwise then the dual becomes way more insightful: maximize ν b T ν A T ν + c 1 Daniel P. Palomar 37

39 Duality for Problems with Generalized Inequalities The Lagrange duality can be naturally extended to generalized inequalities of the form f i (x) Ki 0 where Ki is a generalized inequality on R k i with respect to the cone K i. The corresponding dual variable has to satisfy where K i is the dual cone of K i. λ i K i 0 Daniel P. Palomar 38

40 Semidefinite Programming (SDP) Consider the following SDP (F i, G R k k ): minimize x c T x subject to x 1 F x n F n G. The Lagrange multiplier is a matrix Ψ R k k and the Lagrangian L (x, Ψ) = c T x + Tr (Ψ (x 1 F x n F n G)) The dual problem is maximize Ψ subject to Tr (ΨG) Tr (ΨF i ) + c i = 0, i = 1,..., n Ψ 0. Daniel P. Palomar 39

41 Summary We have introduced the Lagrange duality theory: Lagrangian, dual function, and dual problem. We have developed the optimality conditions for convex problems: the KKT conditions. We have illustrated the used of the KKT conditions to find the closed-form solution to a problem. We have overviewed some additional concepts such as duals of reformulations of problems, sensitivity analysis, generalized inequalities, and SDP. Daniel P. Palomar 40

42 References Chapter 5 of Stephen Boyd and Lieven Vandenberghe, Convex Optimization. Cambridge, U.K.: Cambridge University Press, boyd/cvxbook/ Daniel P. Palomar 41

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