# Duality. Geoff Gordon & Ryan Tibshirani Optimization /

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1 Duality Geoff Gordon & Ryan Tibshirani Optimization /

2 Duality in linear programs Suppose we want to find lower bound on the optimal value in our convex problem, B min x C f(x) E.g., consider the following simple LP min x,y x + y subject to x + y 2 x, y 0 What s a lower bound? Easy, take B = 2 But didn t we get lucky? 2

3 Try again: min x,y x + 3y subject to x + y 2 x, y 0 x + y 2 + 2y 0 = x + 3y 2 Lower bound B = 2 More generally: min x,y px + qy subject to x + y 2 x, y 0 a + b = p a + c = q a, b, c 0 Lower bound B = 2a, for any a, b, c satisfying above 3

4 What s the best we can do? Maximize our lower bound over all possible a, b, c: min x,y px + qy subject to x + y 2 x, y 0 max a,b,c 2a subject to a + b = p a + c = q a, b, c 0 Called primal LP Called dual LP Note: number of dual variables is number of primal constraints 4

5 Try another one: min x,y px + qy subject to x 0 y 1 3x + y = 2 max a,b,c 2c b subject to a + 3c = p b + c = q a, b 0 Primal LP Dual LP Note: in the dual problem, c is unconstrained 5

6 General form LP Given c R n, A R m n, b R m, G R r n, h R r min x R n ct x subject to Ax = b Gx h max u R m,v R r b T u h T v subject to A T u G T v = c v 0 Primal LP Dual LP Explanation: for any u and v 0, and x primal feasible, u T (Ax b) + v T (Gx h) 0, i.e., ( A T u G T v) T x b T u h T v So if c = A T u G T v, we get a bound on primal optimal value 6

7 Max flow and min cut Soviet railway network (from Schrijver (2002), On the history of transportation and maximum flow problems) 7

8 s f ij c ij t Given graph G = (V, E), define flow f ij, (i, j) E to satisfy: f ij 0, (i, j) E f ij c ij, (i, j) E f ik = f kj, k V \{s, t} (i,k) E (k,j) E Max flow problem: find flow that maximizes total value of flow from s to t. I.e., as an LP: max f R E (s,j) E f sj subject to f ij 0, f ij c ij for all (i, j) E f ik = f kj for all k V \ {s, t} (i,k) E (k,j) E 8

9 Derive the dual, in steps: Note that ( ) a ij f ij + b ij (f ij c ij ) (i,j) E + k V \{s,t} x k ( (i,k) E f ik (k,j) E for any a ij, b ij 0, (i, j) E, and x k, k V \ {s, t} Rearrange as (i,j) E M ij (a, b, x)f ij (i,j) E b ij c ij f kj ) 0 where M ij (a, b, x) collects terms multiplying f ij 9

10 10 Want to make LHS in previous inequality equal to primal M sj = b sj a sj + x j want this = 1 objective, i.e., M it = b it a it x i want this = 0 M ij = b ij a ij + x j x i want this = 0 We ve shown that primal optimal value (i,j) E b ij c ij, subject to a, b, x satisfying constraints. Hence dual problem is (minimize over a, b, x to get best upper bound): min b R E,x R V (i,j) E b ij c ij subject to b ij + x j x i 0 b 0, x s = 1, x t = 0 for all (i, j) E

11 11 Suppose that at the solution, it just so happened x i {0, 1} for all i V Call A = {i : x i = 1} and B = {i : x i = 0}, note that s A and t B. Then the constraints b ij x i x j for (i, j) E, b 0 imply that b ij = 1 if i A and j B, and 0 otherwise. Moreover, the objective (i,j) E b ijc ij is the capacity of cut defined by A, B I.e., we ve argued that the dual is the LP relaxation of the min cut problem: min b R E,x R V (i,j) E b ij c ij subject to b ij x i x j b ij, x i, x j {0, 1} for all i, j

12 12 Therefore, from what we know so far: value of max flow optimal value for LP relaxed min cut capacity of min cut Famous result, called max flow min cut theorem: value of max flow through a network is exactly the capacity of the min cut Hence in the above, we get all equalities. In particular, we get that the primal LP and dual LP have exactly the same optimal values, a phenomenon called strong duality How often does this happen? More on this later

13 (From F. Estrada et al. (2004), Spectral embedding and min cut for image segmentation ) 13

14 14 Another perspective on LP duality min x R n ct x subject to Ax = b Gx h max u R m, v R r b T u h T v subject to A T u G T v = c v 0 Primal LP Dual LP Explanation # 2: for any u and v 0, and x primal feasible c T x c T x + u T (Ax b) + v T (Gx h) := L(x, u, v) So if C denotes primal feasible set, f primal optimal value, then for any u and v 0, f min x C L(x, u, v) min L(x, u, v) := g(u, v) x Rn

15 15 In other words, g(u, v) is a lower bound on f for any u and v 0 Note that g(u, v) = { b T u h T v if c = A T u G T v otherwise Now we can maximize g(u, v) over u and v 0 to get the tightest bound, and this gives exactly the dual LP as before This last perspective is actually completely general and applies to arbitrary optimization problems (even nonconvex ones)

16 16 Outline Rest of today: Lagrange dual function Langrange dual problem Examples Weak and strong duality

17 17 Lagrangian Consider general minimization problem min x R n f(x) subject to h i (x) 0, i = 1,... m l j (x) = 0, j = 1,... r Need not be convex, but of course we will pay special attention to convex case We define the Lagrangian as L(x, u, v) = f(x) + m u i h i (x) + i=1 r v j l j (x) j=1 New variables u R m, v R r, with u 0 (implicitly, we define L(x, u, v) = for u < 0)

18 18 Important property: for any u 0 and v, f(x) L(x, u, v) at each feasible x Why? For feasible x, L(x, u, v) = f(x) + m u i h i (x) + }{{} 0 i=1 r j=1 v j l j (x) f(x) }{{} =0 Solid line is f Dashed line is h, hence feasible set [ 0.46, 0.46] Each dotted line shows L(x, u, v) for different choices of u 0 and v (From B & V page 217)

19 19 Lagrange dual function Let C denote primal feasible set, f denote primal optimal value. Minimizing L(x, u, v) over all x R n gives a lower bound: f min x C L(x, u, v) min L(x, u, v) := g(u, v) x Rn We call g(u, v) the Lagrange dual function, and it gives a lower bound on f for any u 0 and v, called dual feasible u, v Dashed horizontal line is f Dual variable λ is (our u) Solid line shows g(λ) (From B & V page 217)

20 20 Quadratic program Consider quadratic program (QP, step up from LP!) where Q 0. Lagrangian: 1 min x R n 2 xt Qx + c T x subject to Ax = b, x 0 L(x, u, v) = 1 2 xt Qx + c T x u T x + v T (Ax b) Lagrange dual function: g(u, v) = min x R n L(x, u, v) = 1 2 (c u+at v) T Q 1 (c u+a T v) b T v For any u 0 and any v, this is lower a bound on primal optimal value f

21 21 Same problem but now Q 0. Lagrangian: 1 min x R n 2 xt Qx + c T x subject to Ax = b, x 0 L(x, u, v) = 1 2 xt Qx + c T x u T x + v T (Ax b) Lagrange dual function: 1 2 (c u + AT v) T Q + (c u + A T v) b T v g(u, v) = if c u + A T v null(q) otherwise where Q + denotes generalized inverse of Q. For any u 0, v, and c u + A T v null(q), g(u, v) is a nontrivial lower bound on f

22 22 Quadratic program in 2D We choose f(x) to be quadratic in 2 variables, subject to x 0. Dual function g(u) is also quadratic in 2 variables, also subject to u 0 primal Dual function g(u) provides a bound on f for every u 0 f / g dual Largest bound this gives us: turns out to be exactly f... coincidence? x1 / u1 x2 / u2 More on this later

23 Given primal problem Lagrange dual problem min x R n f(x) subject to h i (x) 0, i = 1,... m l j (x) = 0, j = 1,... r Our constructed dual function g(u, v) satisfies f g(u, v) for all u 0 and v. Hence best lower bound is given by maximizing g(u, v) over all dual feasible u, v, yielding Lagrange dual problem: max g(u, v) u R m, v Rr subject to u 0 Key property, called weak duality: if dual optimal value g, then f g Note that this always holds (even if primal problem is nonconvex) 23

24 24 Another key property: the dual problem is a convex optimization problem (as written, it is a concave maximization problem) Again, this is always true (even when primal problem is not convex) By definition: { g(u, v) = min f(x) + x R n m u i h i (x) + i=1 { = max f(x) x R n r j=1 m u i h i (x) i=1 } v j l j (x) r j=1 } v j l j (x) }{{} pointwise maximum of convex functions in (u, v) I.e., g is concave in (u, v), and u 0 is a convex constraint, hence dual problem is a concave maximization problem

25 f 25 Nonconvex quartic minimization Define f(x) = x 4 50x x (nonconvex), minimize subject to constraint x 4.5 Primal Dual g x v Dual function g can be derived explicitly (via closed-form equation for roots of a cubic equation). Form of g is quite complicated, and would be hard to tell whether or not g is concave... but it must be!

26 26 Strong duality Recall that we always have f g (weak duality). On the other hand, in some problems we have observed that actually which is called strong duality f = g Slater s condition: if the primal is a convex problem (i.e., f and h 1,... h m are convex, l 1,... l r are affine), and there exists at least one strictly feasible x R n, meaning h 1 (x) < 0,... h m (x) < 0 and l 1 (x) = 0,... l r (x) = 0 then strong duality holds This is a pretty weak condition. (And it can be further refined: need strict inequalities only over functions h i that are not affine)

27 27 Back to where we started For linear programs: Easy to check that the dual of the dual LP is the primal LP Refined version of Slater s condition: strong duality holds for an LP if it is feasible Apply same logic to its dual LP: strong duality holds if it is feasible Hence strong duality holds for LPs, except when both primal and dual are infeasible In other words, we pretty much always have strong duality for LPs

28 28 Mixed strategies for matrix games Setup: two players, vs., and a payout matrix P G R n 1 P 11 P P 1n 2 P 21 P P 2n... m P m1 P m2... P mn Game: if G chooses i and R chooses j, then G must pay R amount P ij (don t feel bad for G this can be positive or negative) They use mixed strategies, i.e., each will first specify a probability distribution, and then x : y : P(G chooses i) = x i, i = 1,... m P(R chooses j) = y j, j = 1,... n

29 29 The expected payout then, from G to R, is m n x i y j P ij = x T P y i=1 j=1 Now suppose that, because G is older and wiser, he will allow R to know his strategy x ahead of time. In this case, R will definitely choose y to maximize x T P y, which results in G paying off max {x T P y : y 0, 1 T y = 1} = max i=1,...n (P T x) i G s best strategy is then to choose his distribution x according to min x R m max i=1,...n (P T x) i subject to x 0, 1 T x = 1

30 30 In an alternate universe, if R were somehow older and wiser than G, then he might allow G to know his strategy y beforehand By the same logic, R s best strategy is to choose his distribution y according to max y R n min j=1,...m (P y) j subject to y 0, 1 T y = 1 Call G s expected payout in first scenario f1, and expected payout in second scenario f2. Because it is clearly advantageous to know the other player s strategy, f1 f 2 We can show using strong duality that f1 = f 2 come as a surprise!... which may

31 Recast first problem as an LP min x R m, t R t subject to x 0, 1 T x = 1 P T x t Lagrangian and Lagrange dual function L(x, u, v, y) = t u T x + v(1 1 T x) + y T (P T x t) { v if 1 1 T y = 0, P y u v = 0 g(u, v, y) = otherwise Hence dual problem is max u R m, t R v subject to y 0, 1 T y = 1 P y v This is exactly the second problem, and we have strong LP duality 31

32 32 References S. Boyd and L. Vandenberghe (2004), Convex Optimization, Cambridge University Press, Chapter 5 R. T. Rockafellar (1970), Convex Analysis, Princeton University Press, Chapters 28 30

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