# 3. Convex functions. basic properties and examples. operations that preserve convexity. the conjugate function. quasiconvex functions

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1 3. Convex functions Convex Optimization Boyd & Vandenberghe basic properties and examples operations that preserve convexity the conjugate function quasiconvex functions log-concave and log-convex functions convexity with respect to generalized inequalities 3 1

2 Definition f : R n R is convex if dom f is a convex set and f(θx +(1 θ)y) θf(x)+(1 θ)f(y) for all x, y dom f, 0 θ 1 (x, f(x)) (y, f(y)) f is concave if f is convex f is strictly convex if dom f is convex and f(θx +(1 θ)y) <θf(x)+(1 θ)f(y) for x, y dom f, x y, 0 <θ<1 Convex functions 3 2

3 Examples on R convex: affine: ax + b on R, foranya, b R exponential: e ax,foranya R powers: x α on R ++,forα 1 or α 0 powers of absolute value: x p on R, forp 1 negative entropy: x log x on R ++ concave: affine: ax + b on R, foranya, b R powers: x α on R ++,for0 α 1 logarithm: log x on R ++ Convex functions 3 3

4 Examples on R n and R m n affine functions are convex and concave; all norms are convex examples on R n affine function f(x) =a T x + b norms: x p =( n i=1 x i p ) 1/p for p 1; x = max k x k examples on R m n (m n matrices) affine function f(x) =tr(a T X)+b = m i=1 n A ij X ij + b j=1 spectral (maximum singular value) norm f(x) = X 2 = σ max (X) =(λ max (X T X)) 1/2 Convex functions 3 4

5 Restriction of a convex function to a line f : R n R is convex if and only if the function g : R R, g(t) =f(x + tv), dom g = {t x + tv dom f} is convex (in t) foranyx dom f, v R n can check convexity of f by checking convexity of functions of one variable example. f : S n R with f(x) = log det X, dom f = S n ++ g(t) = log det(x + tv ) = log det X + log det(i + tx 1/2 VX 1/2 ) = n log det X + log(1 + tλ i ) where λ i are the eigenvalues of X 1/2 VX 1/2 g is concave in t (for any choice of X 0, V ); hence f is concave i=1 Convex functions 3 5

6 Extended-value extension extended-value extension f of f is f(x) =f(x), x dom f, f(x) =, x dom f often simplifies notation; for example, the condition 0 θ 1 = f(θx +(1 θ)y) θ f(x)+(1 θ) f(y) (as an inequality in R { }), means the same as the two conditions dom f is convex for x, y dom f, 0 θ 1 = f(θx +(1 θ)y) θf(x)+(1 θ)f(y) Convex functions 3 6

7 First-order condition f is differentiable if dom f is open and the gradient f(x) = exists at each x dom f ( f(x) x 1, f(x),..., f(x) ) x 2 x n 1st-order condition: differentiable f with convex domain is convex iff f(y) f(x)+ f(x) T (y x) for all x, y dom f f(y) f(x) + f(x) T (y x) (x, f(x)) first-order approximation of f is global underestimator Convex functions 3 7

8 Second-order conditions f is twice differentiable if dom f is open and the Hessian 2 f(x) S n, 2 f(x) ij = 2 f(x) x i x j, i,j =1,...,n, exists at each x dom f 2nd-order conditions: for twice differentiable f with convex domain f is convex if and only if 2 f(x) 0 for all x dom f if 2 f(x) 0 for all x dom f, thenf is strictly convex Convex functions 3 8

9 Examples quadratic function: f(x) =(1/2)x T Px+ q T x + r (with P S n ) f(x) =Px+ q, 2 f(x) =P convex if P 0 least-squares objective: f(x) = Ax b 2 2 f(x) =2A T (Ax b), 2 f(x) =2A T A convex (for any A) quadratic-over-linear: f(x, y) =x 2 /y 2 2 f(x, y) = 2 y 3 [ y x ][ y x ] T 0 f(x, y) convex for y>0 1 y 0 2 x 0 Convex functions 3 9

10 log-sum-exp: f(x) = log n k=1 exp x k is convex 2 f(x) = 1 1 T z diag(z) 1 (1 T z) 2zzT (z k =expx k ) to show 2 f(x) 0, wemustverifythatv T 2 f(x)v 0 for all v: v T 2 f(x)v = ( k z kv 2 k )( k z k) ( k v kz k ) 2 ( k z k) 2 0 since ( k v kz k ) 2 ( k z kv 2 k )( k z k) (from Cauchy-Schwarz inequality) geometric mean: f(x) =( n k=1 x k) 1/n on R n ++ is concave (similar proof as for log-sum-exp) Convex functions 3 10

11 Epigraph and sublevel set α-sublevel set of f : R n R: C α = {x dom f f(x) α} sublevel sets of convex functions are convex (converse is false) epigraph of f : R n R: epi f = {(x, t) R n+1 x dom f, f(x) t} epi f f f is convex if and only if epi f is a convex set Convex functions 3 11

12 Jensen s inequality basic inequality: if f is convex, then for 0 θ 1, f(θx +(1 θ)y) θf(x)+(1 θ)f(y) extension: if f is convex, then f(e z) E f(z) for any random variable z basic inequality is special case with discrete distribution prob(z = x) =θ, prob(z = y) =1 θ Convex functions 3 12

13 Operations that preserve convexity practical methods for establishing convexity of a function 1. verify definition (often simplified by restricting to a line) 2. for twice differentiable functions, show 2 f(x) 0 3. show that f is obtained from simple convex functions by operations that preserve convexity nonnegative weighted sum composition with affine function pointwise maximum and supremum composition minimization perspective Convex functions 3 13

14 Positive weighted sum & composition with affine function nonnegative multiple: αf is convex if f is convex, α 0 sum: f 1 + f 2 convex if f 1,f 2 convex (extends to infinite sums, integrals) composition with affine function: f(ax + b) is convex if f is convex examples log barrier for linear inequalities f(x) = m log(b i a T i x), i=1 dom f = {x a T i x<b i,i=1,...,m} (any) norm of affine function: f(x) = Ax + b Convex functions 3 14

15 Pointwise maximum if f 1,...,f m are convex, then f(x) = max{f 1 (x),...,f m (x)} is convex examples piecewise-linear function: f(x) = max i=1,...,m (a T i x + b i) is convex sum of r largest components of x R n : f(x) =x [1] + x [2] + + x [r] is convex (x [i] is ith largest component of x) proof: f(x) = max{x i1 + x i2 + + x ir 1 i 1 <i 2 < <i r n} Convex functions 3 15

16 Pointwise supremum if f(x, y) is convex in x for each y A,then is convex examples g(x) =supf(x, y) y A support function of a set C: S C (x) =sup y C y T x is convex distance to farthest point in a set C: f(x) =sup x y y C maximum eigenvalue of symmetric matrix: for X S n, λ max (X) = sup y T Xy y 2 =1 Convex functions 3 16

17 Composition with scalar functions composition of g : R n R and h : R R: f(x) =h(g(x)) f is convex if g convex, h convex, h nondecreasing g concave, h convex, h nonincreasing proof (for n =1,differentiableg, h) f (x) =h (g(x))g (x) 2 + h (g(x))g (x) note: monotonicity must hold for extended-value extension h examples exp g(x) is convex if g is convex 1/g(x) is convex if g is concave and positive Convex functions 3 17

18 Vector composition composition of g : R n R k and h : R k R: f(x) =h(g(x)) = h(g 1 (x),g 2 (x),...,g k (x)) f is convex if g i convex, h convex, h nondecreasing in each argument g i concave, h convex, h nonincreasing in each argument proof (for n =1,differentiableg, h) f (x) =g (x) T 2 h(g(x))g (x)+ h(g(x)) T g (x) examples m i=1 log g i(x) is concave if g i are concave and positive log m i=1 exp g i(x) is convex if g i are convex Convex functions 3 18

19 Minimization if f(x, y) is convex in (x, y) and C is a convex set, then is convex examples g(x) = inf f(x, y) y C f(x, y) =x T Ax +2x T By + y T Cy with [ A B B T C ] 0, C 0 minimizing over y gives g(x) =inf y f(x, y) =x T (A BC 1 B T )x g is convex, hence Schur complement A BC 1 B T 0 distance to a set: dist(x, S) =inf y S x y is convex if S is convex Convex functions 3 19

20 Perspective the perspective of a function f : R n R is the function g : R n R R, g(x, t) =tf(x/t), dom g = {(x, t) x/t dom f, t > 0} g is convex if f is convex examples f(x) =x T x is convex; hence g(x, t) =x T x/t is convex for t>0 negative logarithm f(x) = log x is convex; hence relative entropy g(x, t) =t log t t log x is convex on R 2 ++ if f is convex, then g(x) =(c T x + d)f ( (Ax + b)/(c T x + d) ) is convex on {x c T x + d>0, (Ax + b)/(c T x + d) dom f} Convex functions 3 20

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