SEMIDEFINITE PROGRAMMING AND MULTIVARIATE CHEBYSHEV BOUNDS Katherine Comanor ¤ Lieven Vandenberghe ¤¤ Stephen Boyd ¤¤¤ ¤ The RAND Corporation, Santa Monica, California ¤¤ Electrical Engineering Department, UCLA ¤¤¤ Electrical Engineering Department, Stanford University Abstract: Chebyshev inequalities provide bounds on the probability of a set based on known expected values of certain functions, for example, known power moments. In some important cases these bounds can be e±ciently computed via convex optimization. We discuss one particular type of generalized Chebyshev bound, a lower bound on the probability of a set de¯ned by strict quadratic inequalities, given the mean and the covariance of the distribution. We present a semide¯nite programming formulation, give an interpretation of the dual problem, and describe some applications. 1. INTRODUCTION The best lower bound on E f0(X) is given by the optimal value of the linear optimization problem The classical Chebyshev inequality states that minimize E f0(X) (1) subject to E fi(X) = ai; i = 1; : : : ; m; Prob(jXj ¸ 1) · 2 where we optimize over all probability distribu- for a zero-mean random variable X 2 R with vari- tions on Rn. The dual of this problem is ance 2. Generalized Chebyshev bounds provide similar upper or lower bounds on the probability m of a set in Rn based on known expected values maximize z0 + aizi Xi=1 of certain functions, for example, the mean and m (2) covariance. Several such multivariate generaliza- subject to z0 + zifi(x) · f0(x) for all x; tions of Chebyshev's inequality appeared during Xi=1 the 1950s and 1960s, see Isii (1959), Isii (1963), Isii (1964), Marshall and Olkin (1960), Karlin and and has a ¯nite number of variables zi, i = Studden (1966). Isii (1964), for example, considers 0; : : : ; m, but in¯nitely many constraints. Isii the problem of computing upper and lower bounds n shows that strong duality holds under appropriate on E f0(X), where X is a random variable on R constraint quali¯cations, so we can ¯nd a sharp that satis¯es the moment constraints lower bound on E f0(X) by solving (2). Note that the constraints in (2) can be written as a single E fi(X) = ai; i = 1; : : : ; m: constraint g(z0; : : : ; zm) · 0, where m where 1C is the 0-1 indicator function of C (i.e., g(z0; : : : ; zm) = sup(z0 + zifi(x) ¡ f0(x)): 1 (x) = 1 if x 2 C and 1 (x) = 0 otherwise). x C C Xi=1 Hence, if E X = x¹, and E XXT = S, then This is a convex function of z, but in general tr(SP ) + 2qT x¹ + r = E(XT P X + 2qT X + r) di±cult to evaluate. Hence, (2) is usually an intractable optimization problem. ¸ 1 ¡ E 1C (X) Bertsimas and Sethuraman (2000), Lasserre (2002), = 1 ¡ Prob(X 2 C): Bertsimas and Popescu (2005), and Popescu This simple argument shows that 1 ¡ tr(SP ) ¡ (2005) have recently shown that various types of 2qT x¹ ¡ r is a lower bound on Prob(X 2 C). In generalized Chebyshev bounds can be computed the SDP (4) we compute the best lower bound via semide¯nite programming. In this paper we that can be constructed this way. discuss one important example: a lower bound on the probability of a set de¯ned by strict quadratic The proof does not establish that the bound is inequalities, given the mean and the covariance of actually achieved by a distribution with the spec- the distribution. The main purpose of the paper i¯ed moments. This stronger result can be proved is to outline a proof of the main result from by combining Isii's semi-in¯nite linear program- semide¯nite programming duality (see Vanden- ming bounds with the S-procedure (Boyd et al., berghe et al. (2006)), give a geometric interpre- 1994, page 23). It can also be proved directly tation, and describe some applications. from semidefnite programming duality. The dual problem of (4) is m minimize 1 ¡ ¸ 2. PROBABILITY OF A SET DEFINED BY i Xi=1 QUADRATIC INEQUALITIES Zi zi subject to T º 0; i = 1; : : : ; m Rn · zi ¸i ¸ Let C be de¯ned by m strict quadratic (5) inequalities m Zi zi S x¹ T ¹ T T T · zi ¸i ¸ · x¹ 1 ¸ x Aix + 2bi x + ci < 0; i = 1; : : : ; m; (3) Xi=1 T tr(AiZi) + 2bi zi + ci¸i ¸ 0; n with Ai 2 S (not necessarily positive semide¯- i = 1; : : : ; m; n nite), bi 2 R , ci 2 R. It is easily veri¯ed that n n the optimal value of the following semide¯nite which is an SDP with variables Zi 2 S , zi 2 R , program (SDP) is a lower bound on Prob(X 2 C) and ¸i 2 R. It can be shown that from every for distributions with E X = x¹ and E XX T = S: feasible solution in (5) a discrete distribution can be constructed with E X = x¹, E XX T = S and maximize 1 ¡ tr(SP ) ¡ 2qT x¹ ¡ r Prob(X 2 C) · 1 ¡ i ¸i (Vandenberghe et al. P q subject to º 0 (2006)). Tightness of Pthe generalized Chebyshev · qT r ¸ inequality (4) then follows from the fact that the (4) P ¡ ¿iAi q ¡ ¿ibi two SDPs are duals and have the same optimal T º 0; · (q ¡ ¿ibi) r ¡ 1 ¡ ¿ici ¸ value. i = 1; : : : ; m ¿i ¸ 0; i = 1; : : : ; m: 3. GEOMETRIC INTERPRETATION n n The variables in this SDP are P 2 S , q 2 R , 2 Figure 1 shows an example in R . The set C r 2 R, and ¿ 2 R, for i = 1; : : : ; m. i is de¯ned by three linear inequalities and two To verify this, we ¯rst note that the constraints concave quadratic inequalities. The mean x¹ = imply that E X is shown as a small circle, and the set T T T T T T ¡1 x P x + 2q x + r ¸ 1 + ¿i(x Aix + 2bi x + ci) fx j (x ¡ x¹) (S ¡ x¹x¹ ) (x ¡ x¹) = 1g and xT P x + 2qT x + r ¸ 0 for all x. This means is shown as the dashed ellipse. The optimal Cheby- that shev lower bound on Prob(X 2 C) for this exam- ple is 0:3992. The six solid dots are the possible T T x P x + 2q x + r ¸ 1 ¡ 1C (x) values of the discrete distribution computed from maximize 1 ¡ sP ¡ 2xq¹ ¡ r C P q 1 0 0:0311 subject to º ¿ 0:1494 · q r ¡ 1 ¸ · 0 ¡1 ¸ ¿ ¸ 0 PSfrag replacements 0:0527 P q 0:3992 º 0; · q r ¸ x¹ 0:2934 with variables P; q; r; ¿ 2 R. The values P = q = 0:0743 ¿ = 0, r = 1 are obviously feasible, with objective value zero. The values P = ¿ = 1, r = q = 0 are also feasible, with objective value 1¡s. The values T Fig. 1. The distribution that achieves the lower P q 1 1 ¡ x¹ 1 ¡ x¹ = bound on Prob(X 2 C) for a given mean · q r ¸ (s ¡ 2x¹ + 1)2 · s ¡ x¹ ¸ · s ¡ x¹ ¸ and covariance. 1 ¡ x¹ ¿ = s ¡ 2x¹ + 1 the optimal solution of the SDP (5). The numbers next to the dots give the probability of the six are feasible if s < x¹, with objective value values. (Since C is de¯ned as an open set, the 2 ¯ve points on the boundary are not in C itself, so (1 ¡ x¹) 1 ¡ sP ¡ 2xq¹ ¡ r = : Prob(X 2 C) = 0:3992 for this distribution.) s ¡ 2x¹ + 1 The solid ellipse is the level curve This proves (6). To show that the inequality is fx j xT P x + 2qT x + r = 1g tight, we use the dual SDP (5): minimize 1 ¡ ¸ where P , q, and r are the optimal solution of the subject to Z ¸ ¸ lower bound SDP (4). We notice that the optimal Z z s x¹ 0 ¹ ¹ distribution allocates nonzero probability to the · z ¸ ¸ · x¹ 1 ¸ points where the ellipse touches the boundary of C, and to its center. This relation between the so- with variables ¸; Z; z 2 R. If x¹ > s, the values lutions of the upper and lower bound SDPs holds in general, and is a consequence of the optimality s ¡ x¹2 s ¡ x¹2 Z = z = ¸ = = ; conditions of semide¯nite programming. s ¡ 2x¹ + 1 s ¡ x¹2 + (x¹ ¡ 1)2 are feasible with objective function (1 ¡ x¹)2 1 ¡ ¸ = : 4. EXAMPLES s ¡ 2x¹ + 1 In the simplest cases, the two SDPs can be solved If x¹ · s < 1, we can also take Z = s, z = x¹, analytically. As an example, we derive an exten- ¸ = s, which provides a feasible point with a sion of the Chebyshev inequality known as Sel- smaller objective value, 1 ¡ s. Finally, if s ¸ 1, we berg's inequality (Karlin and Studden, 1966, page can take Z = s, z = x¹, ¸ = 1, which has objective 475). We take C = (¡1; 1) = fx 2 R j x2 < 1g. 2 value zero. This shows that there are distributions We show that if E X = x¹ and E X = s, then with the speci¯ed mean and variance for which the righthand side of (6) is equal to Prob(X 2 C). 0 1 · s 2 8 1 ¡ s jx¹j · s < 1 For example, if x¹ = E X = 0:4 and s = E X = Prob(jXj < 1) ¸ 2 (6) > (1 ¡ jx¹j) 0:2 we get Prob(jXj < 1) ¸ 0:9, and the bound < s < jx¹j s ¡ 2jx¹j + 1 is achieved by the distribution > : 1 with probability 0:1 X = and that there is a distribution that achieves the ½ 1=3 with probability 0:9.