Foundations of Reproducing Kernel Hilbert Spaces Advanced Topics in Machine Learning
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1 Foundations of Reproducing Kernel Hilbert Spaces Advanced Topics in Machine Learning slides: teaching.html Gatsby Unit March 22, 2016
2 Overview 1 Concepts from functional analysis: normed-, inner product space, convergent-, Cauchy sequence, complete spaces: Banach-, Hilbert space, continuous/bounded linear operators.
3 Overview 2 RKHS: different views: 1 continuous evaluation functional, 2 reproducing kernel, 3 positive definite function, 4 feature view (kernel). equivalence, explicit construction.
4 Normed space We define the length of a vector. F: vector space over R. : F [0, ) is norm on F, if 1 f = 0 iff. f = 0 (norm separates points),
5 Normed space We define the length of a vector. F: vector space over R. : F [0, ) is norm on F, if 1 f = 0 iff. f = 0 (norm separates points), 2 λf = λ f λ R, f F (positive homogenity),
6 Normed space We define the length of a vector. F: vector space over R. : F [0, ) is norm on F, if 1 f = 0 iff. f = 0 (norm separates points), 2 λf = λ f λ R, f F (positive homogenity), 3 f + g f + g f, g F (triangle inequality).
7 Normed space We define the length of a vector. F: vector space over R. : F [0, ) is norm on F, if 1 f = 0 iff. f = 0 (norm separates points), 2 λf = λ f λ R, f F (positive homogenity), 3 f + g f + g f, g F (triangle inequality). Note: norm metric: d(f, g) = f g study continuity, convergence.
8 Normed space: examples (R, ), ( R d, x p = [ ) i x i p ] 1 p, 1 p. p = 1: x 1 = i x i (Manhattan), p = 2: x 2 = i x i 2 (Euclidean), p = : x = max i x i (maximum norm). ( C[a, b], f p = [ b a f(x) p dx ] 1 p ), 1 p.
9 Inner product space F: vector space over R., : F F R is an inner product on F if for α i R, f i, f, g F 1 α 1 f 1 +α 2 f 2, g = α 1 f 1, g +α 2 f 2, g (linearity),
10 Inner product space F: vector space over R., : F F R is an inner product on F if for α i R, f i, f, g F 1 α 1 f 1 +α 2 f 2, g = α 1 f 1, g +α 2 f 2, g (linearity), 2 f, g = g, f (symmetry),
11 Inner product space F: vector space over R., : F F R is an inner product on F if for α i R, f i, f, g F 1 α 1 f 1 +α 2 f 2, g = α 1 f 1, g +α 2 f 2, g (linearity), 2 f, g = g, f (symmetry), 3 f, f 0; f, f = 0 f = 0.
12 Inner product space F: vector space over R., : F F R is an inner product on F if for α i R, f i, f, g F 1 α 1 f 1 +α 2 f 2, g = α 1 f 1, g +α 2 f 2, g (linearity), 2 f, g = g, f (symmetry), 3 f, f 0; f, f = 0 f = 0. Notes: 1, 2 bilinearity. inner product norm: f = f, f. 1,2,3 ( f, f 0) is called semi-inner product.
13 Inner product space: examples ( R d, x, y = i x iy i ). (R d 1 d 2, A, B F = tr(a T B) = ij A ijb ij ). ( C[a, b], f, g = b a f(x)g(x)dx ).
14 Norm vs inner product Relations: f, g f g (CBS), 4 f, g = f + g 2 f g 2 (polarization identity), f + g 2 + f g 2 = 2 f g 2 (parallelogram law).
15 Norm vs inner product Relations: Notes: f, g f g (CBS), 4 f, g = f + g 2 f g 2 (polarization identity), f + g 2 + f g 2 = 2 f g 2 (parallelogram law). CBS holds for semi-inner products. parallelogram law = characterization of,.
16 Convergent-, Cauchy sequence F: normed space, {f n } n=1 F, f F, Convergent sequence: f n F f if ǫ > 0 N = N(ε) N, s.t. n N, f n f F < ǫ.
17 Convergent-, Cauchy sequence F: normed space, {f n } n=1 F, f F, Convergent sequence: f n F f if ǫ > 0 N = N(ε) N, s.t. n N, f n f F < ǫ. Cauchy sequence: {f n } n=1 is a Cauchy sequence if ǫ > 0 N = N(ε) N, s.t. n, m N, f n f m F < ǫ.
18 Convergent-, Cauchy sequence F: normed space, {f n } n=1 F, f F, Note: Convergent sequence: f n F f if ǫ > 0 N = N(ε) N, s.t. n N, f n f F < ǫ. Cauchy sequence: {f n } n=1 is a Cauchy sequence if ǫ > 0 N = N(ε) N, s.t. n, m N, f n f m F < ǫ. convergent Cauchy: f n f m F f n f F + f f m F.
19 Not every Cauchy sequence converges Examples: 1, 1.4, 1.41, 1.414, ,...: Cauchy in Q, but 2 / Q. ( ) C[0, 1], L 2 [0,1] : 1 f n n 1 n n But a Cauchy sequence is bounded.
20 Banach space, Hilbert space Complete space: Cauchy sequence converges.
21 Banach space, Hilbert space Complete space: Cauchy sequence converges. Banach space = complete normed space, e.g. 1 Let p [1, ), L p (X,A,µ) := { [ } 1/p f : (X,A) R measurable : f p = f(x) dµ(x)] p <. X 2 ( C[a, b], f = max x [a,b] f(x) ).
22 Banach space, Hilbert space Complete space: Cauchy sequence converges. Banach space = complete normed space, e.g. 1 Let p [1, ), L p (X,A,µ) := { [ } 1/p f : (X,A) R measurable : f p = f(x) dµ(x)] p <. ( 2 C[a, b], f = max x [a,b] f(x) ). Hilbert space = complete inner product space; L 2 (X,A,µ). X
23 Linear-, bounded operator F, G: normed spaces. A : F G is called linear operator: 1 A(αf) = α(af) α R, f F, (homogeneity), 2 A(f + g) = Af + Ag f, g F (additivity). G = R: linear functional.
24 Linear-, bounded operator F, G: normed spaces. A : F G is called linear operator: 1 A(αf) = α(af) α R, f F, (homogeneity), 2 A(f + g) = Af + Ag f, g F (additivity). G = R: linear functional. bounded operator: A is linear & A = sup f F Af G f F <. 1 F λ G A : F G {Af : f F 1}
25 Unbounded linear functional: example ( C 1 [0, 1], f := max x [0,1] f(x) ), A(f) = f (0) R: 1 A: linear differentiation & evaluation are linear, 2 f n (x) = e nx (n Z + ): f n 1, but A(f n ) = f n(0) = ne nx x=0 = n = n.
26 Continuous operator Def.: A is continuous at f 0 F: ǫ > 0 δ = δ(ǫ, f 0 ) > 0, s.t. f f 0 F < δ implies Af Af 0 G < ǫ. continuous: if it is continuous at f 0 F.
27 Continuous operator Def.: A is continuous at f 0 F: ǫ > 0 δ = δ(ǫ, f 0 ) > 0, s.t. f f 0 F < δ implies Af Af 0 G < ǫ. continuous: if it is continuous at f 0 F. Example: Let A g (f) := f, g F R, where f, g F. A g is Lipschitz continuous: Ag (f 1 ) A g (f 2 ), F : lin. = f 1 f 2, g F CBS g F f 1 f 2 F.
28 Continuous-bounded relations Theorem: A: linear operator. Equivalent: A is 1 continuous, 2 continuous at one point, 3 bounded.
29 Continuous-bounded relations Theorems: A: linear operator. Equivalent: A is 1 continuous, 2 continuous at one point, 3 bounded. Riesz representation (F: Hilbert, G = R): continuous linear functionals = {, g F : g F}.
30 Let us switch to RKHS-s!
31 View-1: continuous evaluation. Let H R X be a Hilbert space. Consider for fixed x X the δ x : f H f(x) R map.
32 View-1: continuous evaluation. Let H R X be a Hilbert space. Consider for fixed x X the δ x : f H f(x) R map. The (Dirac) evaluation functional is linear: δ x (αf +βg) = (αf +βg)(x) = αf(x)+βg(x) = αδ x (f)+βδ x (g) ( α,β R, f, g H).
33 View-1: continuous evaluation. Let H R X be a Hilbert space. Consider for fixed x X the δ x : f H f(x) R map. The (Dirac) evaluation functional is linear: δ x (αf +βg) = (αf +βg)(x) = αf(x)+βg(x) = αδ x (f)+βδ x (g) ( α,β R, f, g H). Def.: H is called RKHS if δ x is continuous x X.
34 Example for non-continuous δ x H = L 2 [0, 1] f n (x) = x n : 1 f n 0 H since lim f n 0 n 2 = lim n ( 1 0 1/2 x dx) 2n 1 = lim = 0, n 2n+1
35 Example for non-continuous δ x H = L 2 [0, 1] f n (x) = x n : 1 f n 0 H since lim f n 0 n 2 = lim n ( 1 2 but δ 1 (f n ) = 1 δ 1 (0) = /2 x dx) 2n 1 = lim = 0, n 2n+1 In L 2 : norm convergence pointwise convergence.
36 View-1: convergence In RKHS: convergence in norm pointwise convergence! Result: f n H f fn x f.
37 View-1: convergence In RKHS: convergence in norm pointwise convergence! Result: f n H f fn x f. Proof: For any x X, f n (x) f(x) δx def δx lin = δ x (f n ) δ x (f) = δ x (f n f) δ x: bounded δ x f }{{} n f H. }{{} < 0
38 View-2: reproducing elements, kernel trick. Let H be a Hilbert space of X R functions. k : X X R is called a reproducing kernel of H if for x X 1 k(, x) H ( generators ),
39 View-2: reproducing elements, kernel trick. Let H be a Hilbert space of X R functions. k : X X R is called a reproducing kernel of H if for x X, f H 1 k(, x) H ( generators ), 2 f, k(, x) H = f(x) (reproducing property).
40 View-2: reproducing elements, kernel trick. Let H be a Hilbert space of X R functions. k : X X R is called a reproducing kernel of H if for x X, f H 1 k(, x) H ( generators ), 2 f, k(, x) H = f(x) (reproducing property). Specifically: x, y X, k(x, y) = k (, x),k (, y) H.
41 View-2: reproducing elements, kernel trick. Let H be a Hilbert space of X R functions. k : X X R is called a reproducing kernel of H if for x X, f H 1 k(, x) H ( generators ), 2 f, k(, x) H = f(x) (reproducing property). Specifically: x, y X, k(x, y) = k (, x),k (, y) H. Questions Uniqueness, existence?
42 Reproducing kernel: uniqueness Reproducibility & norm definition uniqueness. Let k 1, k 2 be r.k.-s of H. Then for f H, x X f, k 1 (, x) k 2 (, x) H, H lin, k i r.k. = f(x) f(x) = 0.
43 Reproducing kernel: uniqueness Reproducibility & norm definition uniqueness. Let k 1, k 2 be r.k.-s of H. Then for f H, x X f, k 1 (, x) k 2 (, x) H, H lin, k i r.k. = f(x) f(x) = 0. Choosing f = k 1 (, x) k 2 (, x), we get i.e., k 1 = k 2. k 1 (, x) k 2 (, x) 2 H = 0, ( x X)
44 View-2 (r.k.) view-1 (RKHS) Result: H has a r.k. (k) H is a RKHS.
45 View-2 (r.k.) view-1 (RKHS) Result: H has a r.k. (k) H is a RKHS. Proof ( ): δ x (f) k(x, x) f H, i.e. δ x : H R is bounded (hence continuous).
46 View-2 (r.k.) view-1 (RKHS) Result: H has a r.k. (k) H is a RKHS. Proof ( ): δ x (f) k: r.k. = f(x) = f, k(, x) H k(x, x) f H, δx def i.e. δ x : H R is bounded (hence continuous).
47 View-2 (r.k.) view-1 (RKHS) Result: H has a r.k. (k) H is a RKHS. Proof ( ): δ x (f) δx def k: r.k. = f(x) = f, k(, x) H CBS k(, x) H f H k: r.k. = k(x, x) f H, i.e. δ x : H R is bounded (hence continuous).
48 View-2 (r.k.) view-1 (RKHS) Result: H has a r.k. (k) H is a RKHS. Proof ( ): δ x (f) δx def k: r.k. = f(x) = f, k(, x) H CBS k(, x) H f H k: r.k. = k(x, x) f H, i.e. δ x : H R is bounded (hence continuous). Convergence in RKHS uniform convergence! (k: bounded).
49 View-2 (r.k.) view-1 (RKHS):, existence of r.k. Proof ( ): Let δ x be continuous for all x X. 1 By the Riesz repr. theorem f δx H δ x (f) = f, f δx }{{} =k(,x)? H, f H.
50 View-2 (r.k.) view-1 (RKHS):, existence of r.k. Proof ( ): Let δ x be continuous for all x X. 1 By the Riesz repr. theorem f δx H δ x (f) = f, f δx }{{} =k(,x)? H, f H. 2 Let k(x, x) = f δx (x ), x, x X, then k(, x) = f δx H, f, k(, x) H = δ x (f) = f(x). Thus, k is the reproducing kernel.
51 View-3: positive definiteness. Let k : X X R be a symmetric function.
52 View-3: positive definiteness. Let k : X X R be a symmetric function. G := [k(x i, x j )] n i,j=1 : Gram matrix.
53 View-3: positive definiteness. Let k : X X R be a symmetric function. G := [k(x i, x j )] n i,j=1 : Gram matrix. k is called positive definite, if a T Ga 0 for n 1, a R n, (x 1,..., x n ) X n.
54 View-4: kernel as inner product view. Def.: A k : X X R function is called kernel, if 1 φ : X F, where F is a Hilbert space s.t. 2 k(x, y) = φ(x),φ(y) F. Intuition: k is inner product in F.
55 Reproducing kernel kernel positive definiteness Every r.k. is a kernel: φ(x) := k(, x), k(x, y) = k(, x), k(, y) H. Every kernel is positive definite: a T Ga = n i=1 j=1 k def,, F lin = F =, F = n a i a j k(x i, x j ) n a i φ(x i ), i=1 n a i φ(x i ) i=1 2 n a j φ(x j ) j=1 F 0. F
56 Until now Result-1 (proved): RKHS (δ x continuous) reproducing kernel. Result-2 (proved): reproducing kernel kernel positive definite.
57 Until now Result-1 (proved): RKHS (δ x continuous) reproducing kernel. Result-2 (proved): reproducing kernel kernel positive definite. Moore-Aronszajn theorem (follows) positive definite reproducing kernel.
58 Until now Result-1 (proved): RKHS (δ x continuous) reproducing kernel. Result-2 (proved): reproducing kernel kernel positive definite. Moore-Aronszajn theorem (follows) positive definite reproducing kernel. the 4 notions are exactly the same!
59 Moore-Aronszajn construction: high-level view Given: a k : X X R positive definite function. We construct a pre-rkhs H 0 : { } n H 0 = f = α i k(, x i ) : α i R, x i X {k(, x) : x X}, i=1 f, g H0 = k(x, y), where f = k(, x), g = k(, y).
60 Moore-Aronszajn construction: high-level view Given: a k : X X R positive definite function. We construct a pre-rkhs H 0 : { } n H 0 = f = α i k(, x i ) : α i R, x i X {k(, x) : x X}, f, g H0 = n i=1 i=1 j=1 m α i β j k(x i, y j ), where f = n i=1 α ik(, x i ), g = m j=1 β jk(, y j ).
61 Moore-Aronszajn construction: high-level view H 0 will satisfy: 0 linear space ( ); f, g H0 : well-defined & inner product. 1 δ x -s are continuous on H 0 ( x). 2 For any {f n } H 0 Cauchy seq.: f n x 0 f n H 0 0. From H 0 we construct H as: 1 H R X, for which 2 {f n } H 0 -Cauchy seq. such that f n x f.
62 Moore-Aronszajn construction: high-level view Let f, g H := lim n f n, g n H0, (1) x x where f n f, g n g H 0 -Cauchy sequences. H will satisfy: H 0 H: [f n f H 0 ]. H is a RKHS with r.k. k: -1 H: linear space ( ), 0 f, g H : well-defined & inner product. 1 H is complete. 2 δ x-s are continuous on H ( x). 3 H has r.k. k (used to define H 0 ).
63 , H0 : well-defined, k reproducing on H 0 Recall: if f = n i=1 α ik(, x i ), g = m j=1 β jk(, y j ), then f, g H0 = n m α i β j k(x i, y j ). i=1 j=1, H0 is independent of the particular {α i } and {β j }: f, g H0 = n m α i i=1 j=1 β j k(x i, y j ) = n m α i g(x i ) = β j f(y j ). i=1 j=1
64 , H0 : well-defined, k reproducing on H 0 Recall: if f = n i=1 α ik(, x i ), g = m j=1 β jk(, y j ), then f, g H0 = n m α i β j k(x i, y j ). i=1 j=1, H0 is independent of the particular {α i } and {β j }: f, g H0 = n m α i i=1 j=1 β j k(x i, y j ) = reproducing property on H 0 : n m α i g(x i ) = β j f(y j ). i=1 j=1 f, k(, x) H0 = n α i k(x i, x) = f(x). i=1
65 , H0 : inner product The tricky property to check: f H0 := f, f H0 = 0 = f = 0. This holds by CBS (for the semi-inner product, H0 ): x f(x) k r.k. on H 0 = CBS f, k(, x) H0 f H0 k(x, x) = 0. }{{} =0
66 Pre-RKHS: main property-1 δ x is continuous on H 0 ( x): Let f, g H 0, then δ x (f) δ x (g) δx def, k r.k.,, H lin = 0 f g, k(, x) H0 CBS, k r.k. k(x, x) f g H0.
67 Pre-RKHS: main property-2 f n : H 0 -Cauchy ( x) H 0 f 0 n 0: f n : Cauchy bounded, i.e. f n H0 < A. f n : Cauchy n, m N 1 : f n f m H0 < ǫ/(2a). Let f N1 = r i=1 α ik(, x i ). n N 2 : f n (x i ) < ǫ 2r α i For n max(n 1, N 2 ): (i = 1,...,r). f n 2 H 0 <ǫ.
68 Pre-RKHS: main property-2 f n : H 0 -Cauchy ( x) H 0 f 0 n 0: f n : Cauchy bounded, i.e. f n H0 < A. f n : Cauchy n, m N 1 : f n f m H0 < ǫ/(2a). Let f N1 = r i=1 α ik(, x i ). n N 2 : f n (x i ) < ǫ 2r α i (i = 1,...,r). For n max(n 1, N 2 ): f n 2 H 0 = f n, f n H0 f n f N1, f n + f H N1, f n 0 H 0 r f n f H0 N1 f n H0 + α i f n (x i ) <ǫ. }{{}}{{} i=1 <[ǫ/(2a)]a= ǫ ǫ < α i 2r α 2 i
69 , H : well-defined α n = f n, g n H0 is convergent by Cauchyness in R: α n α m <ǫ
70 , H : well-defined α n = f n, g n H0 is convergent by Cauchyness in R: α n α m = f n, g n H0 f m, g m H0 = f n, g n H0 f m, g n H0 + f m, g n H0 f m, g m H0 = f n f m, g n H0 + f m, g n g m H0 f n f m, g n H0 + fm, g n g m H0 g n H0 f n f m H0 + f m H0 g n g m H0 <ǫ. }{{}}{{}}{{}}{{} <A < ǫ <B < ǫ 2A 2B f n, g n : Cauchy bounded, i.e. f n H0 < A, g n H0 < B.
71 , H : well-defined The limit is independent of the Cauchy seq. chosen: let f n, f n x f ; g n, g n x g: H 0 -Cauchy seq.-s, α n = f n, g n H0, α n = f n, g n H0.
72 , H : well-defined The limit is independent of the Cauchy seq. chosen: let f n, f n x f ; g n, g n x g: H 0 -Cauchy seq.-s, α n = f n, g n H0, α n = f n, g n H0. Repeating the previous argument: α n α n g n H0 fn f n }{{} H0 + f H0 n gn g n H0. }{{}}{{}}{{} bounded 0 0 bounded
73 , H : well-defined The limit is independent of the Cauchy seq. chosen: let f n, f n x f ; g n, g n x g: H 0 -Cauchy seq.-s, α n = f n, g n H0, α n = f n, g n H0. Repeating the previous argument: α n α n g n H0 fn f n }{{} H0 + f H0 n gn g n H0. }{{}}{{}}{{} bounded : f n, f n similarly). x f f n f n x 0 f n f n bounded H 0 0 (g n g n
74 , H : inner product The tricky bit: f, f H = 0 f = 0. Let f n x f H 0 -Cauchy, and f, f H = lim n f n 2 H 0 = 0. Then f(x) = lim f n (x) = lim δ x (f n ) ( ) lim δ x f n n n }{{} n H0 = 0, }{{} < 0 ( ): δ x is continuous on H 0.
75 Until now:, H is well-defined & inner product. Remains: 1 δ x -s are continuous on H ( x). 2 H is complete. 3 The reproducing kernel on H is k.
76 δ x -s are continuous on H: lemma H 0 is dense in H. Sufficient to show: f n x f H 0 -Cauchy f n H f.
77 δ x -s are continuous on H: lemma H 0 is dense in H. Sufficient to show: f n x f H 0 -Cauchy f n H f. Proof: Fix ǫ > 0, f n : H 0 -Cauchy N m, n: f m f n H0 < ǫ. Fix n N, then f m f n x f f n. By the definition of H : i.e., f n H f. f f n 2 H = lim m f m f n 2 H 0 ǫ 2,
78 δ x -s are continuous on H Sufficient to show: δ x linear is continuous at f 0. Fix x X. We have seen: δ x is continuous on H 0, i.e. η g 0 H0 = g H0 < η δ x (g) δ x (0) = δ x (g) 0 = g(x) < ǫ/2.
79 δ x -s are continuous on H Sufficient to show: δ x linear is continuous at f 0. Fix x X. We have seen: δ x is continuous on H 0, i.e. η g 0 H0 = g H0 < η δ x (g) δ x (0) = δ x (g) 0 = g(x) < ǫ/2. Take f H: f H < η/2. Since H 0 H dense, f n H 0 -Cauchy, N f(x) f N (x) < ǫ/2 f f N H < η/2 [ f n x f], [ f n H f] f N H0 = f N H f }{{} H + f f N H < η. }{{} < η < η 2 2
80 δ x -s are continuous on H Sufficient to show: δ x linear is continuous at f 0. Fix x X. We have seen: δ x is continuous on H 0, i.e. η g 0 H0 = g H0 < η δ x (g) δ x (0) = δ x (g) 0 = g(x) < ǫ/2. Take f H: f H < η/2. Since H 0 H dense, f n H 0 -Cauchy, N f(x) f N (x) < ǫ/2 f f N H < η/2 [ f n x f], [ f n H f] f N H0 = f N H f }{{} H + f f N H < η. }{{} < η < η 2 2 With g = f N we get f N (x) < ǫ 2 f(x) f(x) f }{{ N(x) + f } N (x) < ǫ. }{{} < ǫ < ǫ 2 2
81 H is complete High-level idea: let {f n } H be any Cauchy seq., f(x) := lim n f n (x) since δ x cont. on H {f n (x)} R Cauchy seq. convergent.
82 H is complete High-level idea: let {f n } H be any Cauchy seq., f(x) := lim n f n (x) since δ x cont. on H {f n (x)} R Cauchy seq. convergent. Question: is the point-wise limit f H?
83 H is complete High-level idea: let {f n } H be any Cauchy seq., f(x) := lim n f n (x) since δ x cont. on H {f n (x)} R Cauchy seq. convergent. Question: is the point-wise limit f H? Idea: 1 H 0 dense in H g n H 0 s.t. g n f n H < 1 n. 2 We show x f; {g n} H 0 : Cauchy seq.} f H. g n f n H f.
84 g n x f : g n (x) f(x) g n (x) f n (x) + f n (x) f(x) = δ x (g n f n ) + f }{{} n (x) f(x). }{{} 0; δ x cont. on H 0; f def.
85 {g n } H 0 is Cauchy sequence: g m g n H0 = g m g n H g m f m H + f m f n H + f n g n H 1 m f m f n }{{ n H. }}{{} 0;f g m,g n def. n:h-cauchy
86 Finally, f n H f : f f n H f g n H + g n f n H g n def. {}}{{}}{ 1 f g n H + n. 0: shown at H 0 dense in H
87 Final property: the reproducing kernel on H is k Let f H, and f n x f H 0 -Cauchy sequence. Then, (a) (b) f, k(, x) H = lim f n, k(, x) n H0 = lim f n (x) (c) = f(x), n where (a): definition of, H, (b): k reproducing kernel on H 0, x (c): f n f.
88 Summary We have shown that RKHS (δ x continuous) reproducing kernel kernel (feature view) positive definite. Moore-Aronszajn theorem: RKHS construction for a k pos. def. function. Idea: 1 pre-rkhs: H 0 = span[{k(, x)} x X], 2 H := pointwise limit of H 0 -Cauchy sequences.
89 Thank you for the attention!
90 Vector space axioms (V,+,λ ) is vector space if [ v 1, v 2, v 3, v V, a, b R]: (v 1 + v 2 )+v 3 = v 1 +(v 2 + v 3 ), (associativity) v 1 + v 2 = v 2 + v 1, (commutativity) 0 : v+0 = v, v : v+( v) = 0, a(bv) = (ab)v, 1v = v, a(v 1 + v 2 ) = av 1 + av 2, (a+b)v = av+bv.
91 H is a vector space H R X Needed: x 1 f H λf H: {f n } H 0 -Cauchy, f n f. {λf n } H 0 ( H 0 : vector space), Cauchy, (λf n )(x) x (λf)(x).
92 H is a vector space H R X Needed: x 1 f H λf H: {f n } H 0 -Cauchy, f n f. {λf n } H 0 ( H 0 : vector space), Cauchy, (λf n )(x) x (λf)(x). 2 f, g H f + g H: {f n },{g n } H 0 -Cauchy, f n x f, g n x g {f n + g n } H 0 ( H 0 : vector space), Cauchy, (f n + g n )(x) x (f + g)(x).
93 , H0 : inner product Needed: for f = i α ik(, x i ), g = j β jk(, y j ) H 0 1 f, g H0 = g, f H0 : f, g H0 = α i β j k(x i, y j ) = β j α i k(y j, x i ) = g, f H0. i j j i
94 , H0 : inner product Needed: for λ R, f = i α ik(, x i ), g = j β jk(, y j ) H 0 1 f, g H0 = g, f H0 : f, g H0 = α i β j k(x i, y j ) = β j α i k(y j, x i ) = g, f H0. i j j i 2 λf, g H0 = λ f, g H0 : λf, g H0 = i,j (λα i )β j k(x i, y j ) = λ i,j α i β j k(x i, y j ) = λ f, g H0.
95 , H0 : inner product Needed: for λ R, f = i α ik(, x i ), g = j β jk(, y j ) H 0 1 f, g H0 = g, f H0 : f, g H0 = α i β j k(x i, y j ) = β j α i k(y j, x i ) = g, f H0. i j j i 2 λf, g H0 = λ f, g H0 : λf, g H0 = i,j (λα i )β j k(x i, y j ) = λ i,j α i β j k(x i, y j ) = λ f, g H0. 3 f 1 + f 2, g H0 = f 1, g H0 + f 2, g H0 [f 1 α i, x i, f 2 α i, x i ]: l.h.s = i,j α i β j k(x i, y j ) = i,j where f 1 + f 2 α i,α i, x i, x i. α i β jk(x i, y j)+ i,j α i β jk(x i, y j ) = r.h.s.,
96 , H0 : inner product Needed: for λ R, f = i α ik(, x i ), g = j β jk(, y j ) H 0 1 f, g H0 = g, f H0 : f, g H0 = α i β j k(x i, y j ) = β j α i k(y j, x i ) = g, f H0. i j j i 2 λf, g H0 = λ f, g H0 : λf, g H0 = i,j (λα i )β j k(x i, y j ) = λ i,j α i β j k(x i, y j ) = λ f, g H0. 3 f 1 + f 2, g H0 = f 1, g H0 + f 2, g H0 [f 1 α i, x i, f 2 α i, x i ]: l.h.s = i,j α i β j k(x i, y j ) = i,j where f 1 + f 2 α i,α 4 f = 0 f, f H0 = 0: i, x i, x i. α i β jk(x i, y j)+ i,j f = 0 k(, x) f, f H0 = 0 0 k(x, x) = 0. α i β jk(x i, y j ) = r.h.s.,
97 , H : semi inner product Needed: for f, f 1, f 2, g H 1 f, g H = g, f H : f, g H = lim n f n, g n H0 H 0 : = lim n g n, f n H0 = g, f H.
98 , H : semi inner product Needed: for f, f 1, f 2, g H,λ R 1 f, g H = g, f H : 2 λf, g H = λ f, g H : f, g H = lim n f n, g n H0 H 0 : = lim n g n, f n H0 = g, f H. λf, g H = lim n λf n, g n H0 H 0 : = lim n λ f n, g n H0 = λ lim n f n, g n H0 = λ f, g H.
99 , H : semi inner product Needed: for f, f 1, f 2, g H,λ R 1 f, g H = g, f H : 2 λf, g H = λ f, g H : f, g H = lim n f n, g n H0 H 0 : = lim n g n, f n H0 = g, f H. λf, g H = lim n λf n, g n H0 H 0 : = lim n λ f n, g n H0 = λ lim n f n, g n H0 = λ f, g H. 3 f 1 + f 2, g H = f 1, g H + f 2, g H : f 1 + f 2, g H = lim n f 1,n + f 2,n, g H0 H 0 : = lim n [ f 1,n, g H0 + f 2,n, g H0 ] = lim n f 1,n, g H0 + lim n f 2,n, g H0 = f 1, g H + f 2, g H.
100 , H : semi inner product Needed: for f, f 1, f 2, g H,λ R 1 f, g H = g, f H : 2 λf, g H = λ f, g H : f, g H = lim n f n, g n H0 H 0 : = lim n g n, f n H0 = g, f H. λf, g H = lim n λf n, g n H0 H 0 : = lim n λ f n, g n H0 = λ lim n f n, g n H0 = λ f, g H. 3 f 1 + f 2, g H = f 1, g H + f 2, g H : f 1 + f 2, g H = lim n f 1,n + f 2,n, g H0 H 0 : = lim n [ f 1,n, g H0 + f 2,n, g H0 ] 4 f = 0 f, f H = 0: Let f n 0 = lim n f 1,n, g H0 + lim n f 2,n, g H0 = f 1, g H + f 2, g H. f, f H = lim n 0, 0 H0 H 0 : = lim n 0 = 0.
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