# Interior-Point Methods

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1 Interior-Point Methods Stephen Wright University of Wisconsin-Madison Simons, Berkeley, August, 2017 Wright (UW-Madison) Interior-Point Methods August / 48

2 Outline Introduction: Problems and Fundamentals Background: Newton s Method, Central Path and Neighborhoods Primal-Dual Path Following for LP: Algorithm and Complexity Extensions: QP, LCP Primal and Dual Barrier methods Relationships between Dual Barrier and Primal-Dual I give few citations (but note that several participants in this program have contributed hugely). The following sources provide more background and bibliographies: [Nesterov and Nemirovskii, 1994], [Wright, 1997], [Ye, 1997], [Vandenberghe, 2016], [Renegar, 2001] Wright (UW-Madison) Interior-Point Methods August / 48

3 Ethos There s extremely elegant theory underlying interior-point methods, that required the development of analysis: e.g. self-concordance, self-scaled barriers and cones, primal-dual potential functions. This theory was instrumental in extensions beyond LP, QP, monotone Linear Complementarity (LCP) to general conic programming, including semidefinite programming (SDP). [Todd, 2001] (SDP review) [Nesterov and Nemirovskii, 1994] (self-concordance, primal barriers, conic programming) [Vandenberghe, 2016] (major elements of theory in slides for class) [Renegar, 2001] (elegant treatment of main algorithmic ideas) However, for LP / QP / LCP, algorithms can be developed and powerful results proved using only very elementary mathematical tools. That s the focus of this talk. Wright (UW-Madison) Interior-Point Methods August / 48

4 Linear Programming (LP) Minimize a linear function in R n over a polyhedron. min c T x s.t. Ax = b, x 0. (P) A is m n real matrix. WLOG assume full rank, so m n. Dual: max z b T z s.t. A T z c. Introduce dual slack variables for equivalent formulation: max z,s b T z s.t. A T z + s = c, s 0. (D) KKT conditions: Ax = b, A T z + s = c, 0 x s 0, (KKT) where means x T s = 0. Thus x i 0, s i 0, and x i s i = 0 for all i. If (x, z, s ) satisfies KKT, then x solves (P) and (z, s ) solves (D). Wright (UW-Madison) Interior-Point Methods August / 48

5 LP Duality Weak Duality: If x is feasible for (P) and (z, s) is feasible for (D), then: c T x b T z. Proof: You ve seen it already this week. Twice! Strong Duality: Exactly three possible scenarios for the primal-dual pair: (i) (P) and (D) both have solutions x and (z, s ), and their objectives are equal at optimality: c T x = b T z ; (ii) One of (P), (D) is unbounded and the other is infeasible; (iii) Both (P) and (D) are infeasible. Note: Don t require a regularity condition (e.g. Slater). Proof: Much trickier! Uses e.g. finite termination of simplex method with anti-cycling rules. Wright (UW-Madison) Interior-Point Methods August / 48

6 Duality and Complementarity If (x, z, s) is primal-dual feasible, all optimality conditions except complementarity are satisfied: Ax = b, A T z + s = c, x 0, s 0, All linear! Easy to place constraints on steps to ensure that all iterates (x k, z k, s k ) remain feasible. For feasible (x, z, s), we have 0 n x i s i = x T s = x T (c A T z) = c T x b T z, i=1 so that c T x b T z. (This is the famous one-line proof of weak duality.) Notation: Measure near-optimality of a feasible triple (x, z, s) by µ = 1 n x T s = 1 n n x i s i. i=1 Wright (UW-Madison) Interior-Point Methods August / 48

7 KKT and Constrained Nonlinear Equations Write KKT conditions as a system of constrained nonlinear equations: Define X = diag(x 1, x 2,..., x n ), S = diag(s 1, s 2,..., s n ), e = (1, 1,..., 1) T, and rewrite KKT as follows: Ax b 0 F 0 (x, z, s) := A z + s c = 0, (x, s) 0. XSe 0 Note that F 0 is a square nonlinear system: F 0 : R 2n+m R 2n+m. Only slightly nonlinear: just the last term, which is x 1 s 1 x 2 s 2 XSe =. = 0. x n s n Wright (UW-Madison) Interior-Point Methods August / 48

8 Primal-Dual Interior-Point Approach Basic primal-dual approach generates a sequence (x k, z k, s k ) of strictly feasible triples, satisfying (note strict inequalities): F := {(x, z, s) : Ax = b, A T z + s = c, x > 0, s > 0}. Steps are Newton-like steps on F 0 : (x k+1, z k+1, s k+1 ) = (x k, z k, s k )+α k ( x k, z k, s k ), some α k > 0. Iterate toward complementarity, hence optimality: µ k := 1 n (x k ) T s k 0 as k. Keep the pairwise products x k i s k i, i = 1, 2,..., n more or less balanced not too far from their average value µ k. (Explicitly or implicity.) Wright (UW-Madison) Interior-Point Methods August / 48

9 Complexity Polynomial complexity follows from a decrease per iteration of the form µ k+1 (1 Cn ν )µ k, k = 0, 1, 2,..., where C > 0 is independent of n and ν.5 for conventional methods. Such rates require delicate manipulation of the modified Newton strategy for computing steps, and also of the choice of step length α k. They lead to complexity results via a standard argument: µ K ɛµ 0 (1 Cn ν ) K ɛ K log(1 Cn ν ) log ɛ K( Cn ν ) log ɛ K n ν log ɛ /C. since log(1 + t) t Thus get µ K /µ 0 ɛ in O(n ν log ɛ ) iterations. Wright (UW-Madison) Interior-Point Methods August / 48

10 Newton s Method for Nonlinear Algebraic Equations Consider F : R N R N smooth. We wish to solve F (w) = 0. Basic Idea: At some iterate w, one form of Taylor s theorem implies F (w + d) = F (w) + J(w)d + o( d ), where o( d )/ d 0 as d 0, and J(w) is the Jacobian (N N matrix of first partial derivatives): [ ] Fi J(w) = (w). w j i = 1, 2,..., n j = 1, 2,..., n If J(w) is nonsingular, Newton step is d = J(w) 1 F (w). From expansion above we get for this d: F (w + d) = o( J(w) 1 F (w) ). If the smallest singular value of J(w) is uniformly bounded below on some region (as in the neighborhood of a nondegenerate solution w ), have F (w + d) = o( F (w) ). Wright (UW-Madison) Interior-Point Methods August / 48

11 Newton s Method in 1D This suggests an iterative scheme: Starting from some w 0 R N : d k = J(w k ) 1 F (w k ), w k+1 = w k + d k, k = 0, 1, 2,.... F(w) w * w 2 w 1 w 0 w Wright (UW-Madison) Interior-Point Methods August / 48

12 Newton s Method for Minimization There is also a Newton s method for minimization: min x R N f (x), where f : R N R is smooth (typically twice Lipschitz continuously differentiable). Motivated again by Taylor s theorem, using the approximation f (x + d) f (x) + f (x) T d d T 2 f (x)d. When 2 f (x) is positive definite, the minimizer is d = 2 f (x) 1 f (x).... the same step we get by applying nonlinear-equations Newton s method to F (x) = f (x). Local quadratic convergence to nondegenerate minimizers x, for which f (x ) = 0 and 2 f (x ) is positive definite. Wright (UW-Madison) Interior-Point Methods August / 48

13 Newton s Method for Minimization By substituting the Newton step d = 2 f (x) 1 f (x) into the quadratic approximation, obtain a Newton decrement f (x + d) f (x) 1 2 f (x)t 2 f (x) 1 f (x) = f (x) 1 2 d T 2 f (x)d. Introduce notation for the decrement: λ(x) := f (x) x = d x, where v x := ( 1/2 ( ) 1/2 v T 2 f (x) v) 1, w x := d T 2 f (x)d are Hessian-weighted norms [Vandenberghe, 2016]. We can make stronger statements about the decrement when f is self-concordant. It plays an important role in the analysis of primal barrier methods. Wright (UW-Madison) Interior-Point Methods August / 48

14 Nonlocal Results? Do we need them? Under certain assumptions, Newton s method converges locally at a quadratic rate. What happens outside the domain of quadraticness? Are global rates of convergence available? Leading to complexity bounds? In general, no. For some functions Newton is as slow as first-order methods. But for self-concordant functions, yes! These have 3rd derivatives along any direction bounded in terms of 2nd derivatives, so the quadratic approximation has guaranteed quality everywhere, and a scaled Newton step makes significant progress. For primal-dual methods for LP, we can specialize the analysis to the particular form of F (perturbed KKT conditions). Wright (UW-Madison) Interior-Point Methods August / 48

15 Primal-Dual Path-Following Path-following primal-dual interior-point methods generate search directions from perturbations of the KKT system F 0, and choose step lengths α k with reference to central path neighborhoods. The Central Path is a trajectory in primal-dual space defined by a scalar-parametrized variant of the KKT system: For any τ > 0 let (x τ, z τ, s τ ) be the solution of Ax b 0 F τ (x, z, s) := A T z + s c = 0, (x, s) > 0. XSe τe 0 Central path is C := {(x τ, z τ, s τ ) : τ > 0}. Strictly feasible set F nonempty (x τ, z τ, s τ ) is defined uniquely for each τ > 0. Wright (UW-Madison) Interior-Point Methods August / 48

16 Central Path Neighborhoods Require more than strict feasibility of iterates, which is (x k, s k ) > 0. Require iterates to stay within certain neighborhoods of the central path. Want the pairwise products x i s i to be not too different for i = 1, 2,..., n. N 2 (θ) := {(x, z, s) F : XSe µe 2 θµ, µ = x T s/n}, for some θ (0, 1). Typically θ =.25 or.5. N (γ) := {(x, z, s) F : x i s i γµ, µ = x T s/n, i = 1, 2,..., n}, for some γ (0, 1). Typically γ =.001. The N neighborhood is wider than N 2. Note that N (γ) F as γ 0. (But N 2 (θ) F as θ 1.) Wright (UW-Madison) Interior-Point Methods August / 48

17 Path-Following Strategy Define a target point on the central path: (x(τ), z(τ), s(τ)) for some τ > 0 that depends on the current iterate; Calculate a Newton step for F τ from the current iterate; Choose step length α k along the Newton step to stay inside a central path neighborhood. We describe a long-step path-following algorithm (LPF) that starts from a point (x 0, z 0, s 0 ) N (γ) for some γ (0, 1) and achieves a geometric decrease per iteration in µ: µ k+1 (1 δn 1 )µ k, k = 0, 1, 2,..., for δ independent of n. Thus we have It s also fast in practice. µ k ɛµ 0 in O(n log ɛ) iterations. Wright (UW-Madison) Interior-Point Methods August / 48

18 Long-Step Path Following Notation: µ k = (x k ) T s k /n, (x k (α), z k (α), s k (α)) = (x k, z k, s k ) + α( x k, z k, s k ). Algorithm LPF. Choose γ (0, 1), σ min and σ max with 0 < σ min < σ max < 1; Choose (x 0, z 0, s 0 ) N (γ); for k = 0, 1, 2,... do Choose σ k [σ min, σ max ]; Let ( x k, z k, s k ) be the Newton step for F σk µ k at (x k, z k, s k ); Choose α k to be the largest value in (0, 1] for which (x k (α), z k (α), s k (α)) N (γ); Set (x k+1, z k+1, s k+1 ) = (x k (α k ), z k (α k ), s k (α k )). end for Wright (UW-Madison) Interior-Point Methods August / 48

19 The Search Direction The Newton step for F σk µ k at (x, z, s) satisfies (omitting superscripts): 0 A T I x 0 A 0 0 z = 0. S 0 X s XSe + σ k µ k e Decomposing the third block into individual components, we have s i x i + x i s i = x i s i + σ k µ k. Summing both sides over i = 1, 2,..., n, obtain s T x + x T s = x T s + σ k nµ k = (1 σ k )nµ k The difference between (x + x, z + z, s + s) and the solution of F σk µ k (x, z, s) = 0 is only the term X Se in the third block, where X = diag( x 1, x 2,..., x n ), S = diag( s 1, s 2,..., s n ). The analysis is mostly concerned with showing that this term is not too large, that is, the linear approximation to F σk µ k that is the basis of Newton s method is good enough to make substantial progress. Wright (UW-Madison) Interior-Point Methods August / 48

20 Can plot progress of the algorithm in xs space: x 2 s2 1 0 C 2 3 N 8 (γ) x s 1 1 Wright (UW-Madison) Interior-Point Methods August / 48

21 Convergence and Complexity in Five Easy Steps The result follow from five technical but elementary claims, each building on the one before: 1 If u, v are two vectors in R n with u T v 0, we have UVe 2 3/2 u + v 2, where U = (u 1, u 2,..., u n ), V = (v 1, v 2,..., v n ) X Se 2 3/2 (XS) 1/2 ( XSe + σµe) 2. X Se 2 3/2 (1 + γ 1 )nµ. (x k (α), z k (α), s k (α)) N (γ) for all α [ 0, 2 3/2 γ 1 γ ] σ. 1 + γ n 5 µ k+1 (1 δn 1 )µ k for some δ independent of n and all k 0. Wright (UW-Madison) Interior-Point Methods August / 48

22 Proof of 1 For scalars α and β with αβ 0, we have αβ = αβ (α + β) 2 /4. Define P = {i : u i v i 0}, N := {i : u i v i < 0}. Since u T v 0, we have 0 u T v = i P u i v i + i N u i v i = [u i v i ] i P 1 [u i v i ] i N 1. Thus UVe = ( [u i v i ] i P 2 + [u i v i ] i N 2) 1/2 ( 1/2 [u i v i ] i P [u iv i ] i N 1) 2 since 2 1 ( 1/2 2 [u i v i ] i P 1) 2 by the bound above [ ] (u i + v i ) 2 from the α/β bound = 2 3/2 i P i P 1 n (u i + v i ) 2 2 3/2 (u i + v i ) 2 = 2 3/2 u + v 2. i=1 Wright (UW-Madison) Interior-Point Methods August / 48

23 Proof of 2 Multiply last block of Newton equations by (XS) 1/2, and define D = X 1/2 S 1/2 to obtain S x + X s = XSe + σµe D 1 x + D s = (XS) 1/2 ( XSe + σµe). Set u = D 1 x and v = D s and note from first two blocks of Newton equations that u T v = x T s = x T A T z = (A x) T z = 0. Thus can apply 1 to deduce that X Se 2 3/2 (XS) 1/2 ( XSe + σµe) 2. Wright (UW-Madison) Interior-Point Methods August / 48

24 Proof of 3 Expand the right-hand side of 2: X Se 2 3/2 (XS) 1/2 e + σµ(xs) 1/2 e 2 [ ] n 2 3/2 x T s 2σµe T e + σ 2 µ 2 1 x i s i i=1 [ 2 3/2 x T s 2σµe T e + σ 2 µ 2 n ] since x i s i γµ γµ [ ] 2 3/2 1 2σ + σ2 nµ since e T e = n γ 2 3/2 (1 + γ 1 )nµ since σ (0, 1). Wright (UW-Madison) Interior-Point Methods August / 48

25 Proof of 4 From 3 we have x i s i X Se 2 2 3/2 (1 + γ 1 )nµ. Thus from x i s i γµ and the third block of the Newton equations, we have x i (α)s i (α) = (x i + α x i )(s i + α s i ) = x i s i + α(x i s i + s i x i ) + α 2 x i s i x i s i (1 α) + ασµ α 2 x i s i γ(1 α)µ + ασµ α 2 2 3/2 (1 + γ 1 )nµ. Meanwhile, again using the third block of Newton equations, we have nµ(α) = x(α) T s(α) = (x + α x) T (s + α s) = x T s + α(s T x + x T s) + α 2 x T s = (1 α + ασ)nµ. Wright (UW-Madison) Interior-Point Methods August / 48

26 So for x i (α)s i (α) γµ(α), it suffices to have γ(1 α)µ + ασµ α 2 2 3/2 (1 + γ 1 )nµ γ(1 α + ασ)µ, which is equivalent to proving 4. α 2 3/2 γ 1 γ σ 1 + γ n, Wright (UW-Madison) Interior-Point Methods August / 48

27 Proof of 5 We showed already that µ k (α) = (1 α(1 σ k ))µ k, so that µ k (α) is decreasing in α over α [0, 1]. From 4, we have for step actually taken that Thus α k 2 3/2 γ 1 γ σ 1 + γ n. ( µ k+1 = µ k (α k ) µ k (1 2 3/2 γ 1 γ )) σ k 1 + γ n (1 σ k) µ k. Recalling that σ k [σ min, σ max ] for all k, we have that µ k+1 (1 δn 1 )µ k, for δ = 2 3/2 γ n 1 γ 1 + γ min(σ min(1 σ min ), σ max (1 σ max )). Wright (UW-Madison) Interior-Point Methods August / 48

28 Other Path-Following Algorithms: SPF Short-step Path-Following (SPF) uses the more restricted neighborhood N 2 (0.4), fixed σ = 1 0.4/ n, and full modified Newton steps. Algorithm SPF. Set σ = 1 0.4/ n; Choose (x 0, z 0, s 0 ) N 2 (0.4); for k = 0, 1, 2,... do Let ( x k, z k, s k ) be the Newton step for F σµk at (x k, z k, s k ); Set (x k+1, z k+1, s k+1 ) = (x k, z k, s k ) + ( x k, z k, s k ); end for Analysis is similar to LPF, leading to a better dependence on n: µ k+1 = (1 0.4/ n)µ k, thus achieving µ k ɛµ 0 in O( n log ɛ) iterations. Better complexity bound, but slower in practice. (That s a theme!) Wright (UW-Madison) Interior-Point Methods August / 48

29 Other Path-Following Algorithms: Predictor-Corrector Use two neighborhoods N 2 (.25) and N 2 (.5). Alternate between Predictor steps with σ = 0 (unmodified Newton on KKT) that start in N 2 (.25) and stop at the boundary of N 2 (.5); Corrector steps with σ = 1 that return to N 2 (.25). Algorithm PC. Choose (x 0, z 0, s 0 ) N 2 (0.25); for k = 0, 1, 2,... do if k even then Let ( x k, z k, s k ) be the Newton step for F 0 at (x k, z k, s k ); Choose α k to be the largest value in (0, 1] for which (x k (α), z k (α), s k (α)) N 2 (0.5); Set (x k+1, z k+1, s k+1 ) = (x k (α k ), z k (α k ), s k (α k )); else Let ( x k, z k, s k ) be the Newton step for F µk at (x k, z k, s k ); Set (x k+1, z k+1, s k+1 ) = (x k, z k, s k ) + ( x k, z k, s k ); end if end Wright for (UW-Madison) Interior-Point Methods August / 48

30 PC in xs space 2 x 2 s2 N (.25) 2 N (.5) 2 0 C x s 1 1 Analysis and complexity is similar to SPF: O( n log ɛ) iterations. Wright (UW-Madison) Interior-Point Methods August / 48

31 Primal-Dual Potential Reduction An alternative way to balance centrality of (x k, z k, s k ) while steadily reducing duality gap µ k to zero is by using a primal-dual logoarithmic potential function: for ρ > n. Note that Φ ρ (x, s) := (ρ n) log µ Φ ρ (x, s) := ρ log x T s n i=1 n log x i s i, i=1 log x is i µ + ρ log n, where µ = x T s/n, so reducing ρ and maintaining centrality are both represented. All iterates are strictly feasible: (x k, z k, s k ) F. Search directions are modified Newton steps for F σµk, with σ n/ρ. Choose step length α to minimize Φ ρ along (x k, z k, s k ) + α( x k, z k, s k ); Wright (UW-Madison) Interior-Point Methods August / 48

32 Algorithm PDPR Algorithm PDPR. Choose ρ > n, σ = n/ρ, and (x 0, z 0, s 0 ) F ; for k = 0, 1, 2,... do Let ( x k, z k, s k ) be the Newton step for F σµk at (x k, z k, s k ); Set α max = largest value of α s.t. (x k (α), z k (α), s k (α)) F ; Set α k = arg min Φ ρ(x k (α), s k (α)); α (0,α max) Set (x k+1, z k+1, s k+1 ) = (x k (α k ), z k (α k ), s k (α k )); end for Wright (UW-Madison) Interior-Point Methods August / 48

33 Complexity of PDPR For ρ n + n, we have Φ ρ (x k+1, s k+1 ) Φ ρ (x k, s k ) 0.15, k = 0, 1, 2,..., so that Φ ρ (x k, s k ) at a steady rate. Can show that µ exp (Φ ρ (x, s)/(ρ n)), so we have µ k ɛ in Φ ρ (x 0, s 0 ) + (ρ n) log ɛ δ iterations. Wright (UW-Madison) Interior-Point Methods August / 48

34 Initialization The description of LPF assumes that we can find an initial point (x 0, z 0, s 0 ) N (γ). This is nontrivial in general. Two approaches: Modify the algorithm to not require feasiblity of the equality constraints Ax = b and A T z + s = c. Infeasible-interior-point algorithms. Modify the LP formulation so that a feasible point is obvious. Homogeneous self-dual (HSD) formulations. Solved using interior-point methods for monotone linear complementarity (see later). In implementations, infeasible-interior-point is preferred, and the analysis above can be extended to show convergence and complexity, at slightly worse rates: O(n 2 log ɛ) instead of O(n log ɛ). But HSD is fairly practical, and has elegant theory. Wright (UW-Madison) Interior-Point Methods August / 48

35 Infeasible-Interior-Point Start from any point (x 0, z 0, s 0 ) with (x 0, s 0 ) > 0. For any iterate (x k, z k, s k ), define residuals r k b := Ax k b, r k c := A T z k + s k c. Search directions are still Newton steps for F σk µ k at (x k, z k, s k ), now defined by these Newton equations: 0 A T I x k r k A 0 0 z k c = r k S k 0 X k s k b. X k S k e + σ k µ k e No longer have the nice relation ( x k ) T s k = 0, so µ does not decrease linearly in steplength α along ( x k, z k, s k ). The residual norms r b and r c do however decrease linearly in α, since the first two blocks of the Newton system are linear. Wright (UW-Madison) Interior-Point Methods August / 48

36 Algorithm IPF Infeasible extension of the N central path neighborhood: N (γ, β) := {(x, z, s) : (r b, r c ) β (r 0 b, r 0 c ) (µ/µ 0 ), x i s i γµ}, where µ = x T s/n as usual, and γ (0, 1) and β 1. Algorithm IPF. Choose γ (0, 1), β 1, σ min and σ max with 0 < σ min < σ max < 1; Choose (x 0, z 0, s 0 ) with (x 0, s 0 ) > 0; for k = 0, 1, 2,... do Choose σ k [σ min, σ max ]; Let ( x k, z k, s k ) be the Newton step for F σk µ k at (x k, z k, s k ); Choose α k to be the largest value in (0, 1] for which and (x k (α), z k (α), s k (α)) N (γ, β); µ k (α) (1.01α)µ k ; Set (x k+1, z k+1, s k+1 ) = (x k (α k ), z k (α k ), s k (α k )). end for Wright (UW-Madison) Interior-Point Methods August / 48

37 Convergence of IPF Infeasibility complicates the analysis considerably, but still requires only elementary tools. Ultimately, show that α k δ/n 2 for some δ independent of n and all k. Thus get µ k ɛµ 0 and (r k b, r k c ) βɛ (r 0 b, r 0 c ) in O(n 2 log ɛ) iterations. Wright (UW-Madison) Interior-Point Methods August / 48

38 Extension: Monotone Linear Complementarity Given M R n n positive semidefinite and q R n, seek (x, s) such that s = Mx + q, (x, s) 0, x T s = 0. (LCP) Can add extra unconstrained variables and equality constraints to get a mixed monotone LCP. KKT conditions for LP and convex QP are mixed monotone LCP. For LP: [ ] [ ] [ ] [ ] s 0 A T x c = +, (x, s) 0, x T s = 0. 0 A 0 z b For convex QP problem min 1 2 x T Qx + c T x s.t. Ax = b, x 0, the corresponding LCP is [ ] [ ] [ ] [ ] s Q A T x c = +, 0 A 0 z b (x, s) 0, x T s = 0. Wright (UW-Madison) Interior-Point Methods August / 48

39 Interior-Point Algorithms for LCP Algorithms for monotone LCP are almost identical to those for LP. It is an appealing framework because notation is simpler just (x, s); can be extended immediately to mixed montone LCP; apply to convex QP as well as LP, and also other LCP applications e.g. from games. The analysis is a little different from LP because we have only x T s 0 instead of x T s = 0, but otherwise algorithms, convergence analysis, complexity results are much the same. Wright (UW-Madison) Interior-Point Methods August / 48

40 LP: Homogeneous Self-Dual Formulation Returning to LP: We can derive a mixed monotone LCP from the KKT conditions for LP, such that It s easy to find a strictly feasible initial point for the mixed LCP; The mixed LCP always has a solution; This solution yields either a primal-dual solution pair for the LP, or a certificate of infeasibility. Given (x 0, z 0, s 0 ) such that (x 0, s 0 ) > 0 define b := b Ax 0, c := c A T z 0 s 0, t := c T x b T z 0. (The first two represent initial infeasibilities.) The mixed LCP is then κ 0 c T b T t τ 0 s 0 = c 0 A T c x b A 0 b z + 0 0, 0 t c T b T 0 θ (x 0 ) T s τ κ 0, 0 x s 0. Wright (UW-Madison) Interior-Point Methods August / 48

41 HSD Results Note that the original LP variables appear alongside scalar auxiliary variables κ, τ, θ. A strictly feasible initial point for the mixed LCP is (τ, x, z, θ, κ, s) = (1, x 0, z 0, 1, 1, s 0 ). The term homogeneous self-dual (HSD) arises from the mixed LCP being reduced KKT conditions for a self-dual LP formulation that looks quite similar and that is homogeneous except for the term (x 0 ) T s Result 0: The mixed LCP has a strictly complementary solution. This follows from the close relationship of the mixed LCP to the HSD linear program the fact that the HSD LP is feasible when a primal-dual pair of LPs has a solution, it has a strictly complementary solution. Wright (UW-Madison) Interior-Point Methods August / 48

42 HSD Results Result 1: Any solution (τ, x, z, θ, κ, s ) of the mixed LCP has θ = 0. (Proof: Elementary) Result 2: The original LP has primal-dual solutions if and only if all strictly complementary solutions of the mixed LCP have κ = 0 and τ > 0. These solutions are x /τ for the primal and (z /τ, s /τ ) for the dual. Result 3: If the mixed LCP has a strictly complementary solution for which κ > 0, then at least one of c T x and b T z is negative, and if c T x < 0, then the dual LP is infeasible; if b T z < 0, then the primal LP is infeasible. Wright (UW-Madison) Interior-Point Methods August / 48

43 Log Barrier Functions for LP We turn now to a more traditional approach based on log barrier functions [Frisch, 1955]. Deal with an algebraic constraint c i (x) 0 by adding a term log c i (x) into the objective, weighted by a parameter. Log is defined only when the constraint is strictly feasible: c i (x) > 0; Goes to as c i (x) 0. It is also self-concordant, which allows non-local results to be proved about convergence of Newton s method applied to the barrier formulation. For LP, we have barrier formulations of P and D, parametrized by τ > 0: 1 min x µ ct x max z n log x i s.t. Ax = b, (P-µ) i=1 1 µ bt z + m log(c i A T i z). i=1 (D-µ) Wright (UW-Madison) Interior-Point Methods August / 48

44 Optimality Conditions, Central Path KKT conditions for (P-µ) are: c µx 1 e A T z = 0, Ax = b, where as before X = diag(x 1, x 2,..., x n ) and e = (1, 1,..., 1) T. Defining s i = µ/x i, i = 1, 2,..., n, we can rewrite these conditions as: c s A T z = 0, Ax = b, XSe = µe, which are exactly the conditions that define the point (x µ, z µ, s µ ) on the central path C! Thus the solutions of (P-µ) for µ > 0 are exactly the projections of primal-dual central path onto x-space. A similar derivation works for the dual: The solutions of (D-µ) for µ > 0 are exactly the projection of C into z-space. (Here we set s := c A T z and x i := µ/s i.) Wright (UW-Madison) Interior-Point Methods August / 48

45 Path-Following Thus we can solve (D) by following the path of solutions of (D-µ) as µ 0. The strategy is basically the same as in primal-dual methods: Take Newton steps for (D-µ) at current µ, with steplengths chosen to maintain strict feasibility of iterates, until a near-solution for (D-µ) is found; Decrease µ by some factor σ < 1. The same strategy can be applied to (P-µ), except that here we take Lagrange-Newton steps, because of the constraint Ax = b. Complexity results follow from: Only one or a few Newton steps needed for each µ; (1 σ) 1 log ɛ outer iterations to achieve µ/µ 0 < ɛ. Wright (UW-Madison) Interior-Point Methods August / 48

46 Newton Directions: Dual Barrier and Primal-Dual Defining s := c A T z, Newton equations for (D-τ) are AS 2 A T z = 1 τ b + AS 1 e. We can unpack these equations to make a direct comparison with the primal-dual equations. Suppose that the current s is the exact solution of (D-µ). Then for x := µs 1 e, we have so the system above is b = Ax = µas 1 e, AS 2 A T z = ( µ τ + 1 ) AS 1 e. Choosing τ = σµ as in primal-dual, we find that z is part of the solution of the unpacked system 0 A T I x 0 A 0 0 z = 0 S 0 X s µ ( 1 1 ). σ e Wright (UW-Madison) Interior-Point Methods August / 48

47 Primal-Dual vs Dual Barrier Compare the dual system 0 A T I x 0 A 0 0 z = 0 S 0 X s µ ( 1 1 ). σ e with the primal-dual system from the point (x µ, z µ, s µ ): 0 A T I x 0 0 A 0 0 z = 0 = 0. S 0 X s XSe + σµe µ( 1 + σ)e The RHS coefficients are: Dual Barrier: σ 1, Primal-Dual: σ 1. σ Wright (UW-Madison) Interior-Point Methods August / 48

48 Primal-Dual vs Dual Barrier When σ is close to 1, there is little difference. But when σ is significantly less than 1 (as in long-step methods), the dual barrier step is much longer. It overshoots the solution of (D-σµ). However, the directions z and z are the same, modulo a scaling. Thus we can recover good behavior of dual barrier for long step methods by ensuring accurate enough solution of each subproblem (D-µ); scaling the first step taken after each resetting of the target to σµ. [Wright and Jarre, 1998] Wright (UW-Madison) Interior-Point Methods August / 48

49 References I Frisch, K. R. (1955). The logarithmic potential method of convex programming. Technical Report, University Institute of Economics, Oslo, Norway. Nesterov, Y. and Nemirovskii, A. S. (1994). Interior Point Polynomial Methods in Convex Programming. SIAM Publications, Philadelphia. Renegar, J. (2001). A Mathematical View of Interior-Point Methods in Convex Optimization. MPS-SIAM Series in Optimization. SIAM. Todd, M. J. (2001). Semidefinite optimization. Acta Numerica, 10: Vandenberghe, L. (2016). Class notes for ee236c. vandenbe/236c/lectures/. Wright, S. J. (1997). Primal-Dual Interior-Point Methods. SIAM, Philadelphia, PA. Wright (UW-Madison) Interior-Point Methods August / 2

50 References II Wright, S. J. and Jarre, F. (1998). The role of linear objective functions in barrier methods. Mathematical Programming, Series A, 84: Ye, Y. (1997). Interior Point Algorithms : Theory and Analysis. Wiley-Interscience Series in Discrete Mathematics and Optimization. John Wiley and Sons. Wright (UW-Madison) Interior-Point Methods August / 2

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