My Favorite Theorem: The Reisz Representation Theorem Fred J. Hickernell Department of Applied Mathematics Center for Interdisciplinary Scientific Computation Office of Research Illinois Institute of Technology hickernell@iit.edu mypages.iit.edu/~hickernell Thanks to the Illinois Tech SIAM Student Chapter for the invitation Thanks to many students and collaborators for teaching me Slides available at speakerdeck.com/riesz-representation-theorem Please interrupt and ask questions October 19, 2021

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Main Idea What seems obvious for two-dimensional vectors becomes a power tool for numerical analysis 2/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Main Idea What seems obvious for two-dimensional vectors becomes a power tool for numerical analysis Obvious If LINEAR : R2 → R is any linear, real-valued function, meaning, LINEAR(cf + h) = c LINEAR(f) + LINEAR(h) ∀f, h ∈ R2, c ∈ R, then LINEAR(f) can be represented as an inner product: ∃ coefficient g ∈ R2 such that LINEAR(f) = g1 f1 + g2 f2 = gTf =: ⟨g, f⟩ ≡ g • f ∀f ∈ R2. 2/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Main Idea What seems obvious for two-dimensional vectors becomes a power tool for numerical analysis Obvious If LINEAR : R2 → R is any linear, real-valued function, meaning, LINEAR(cf + h) = c LINEAR(f) + LINEAR(h) ∀f, h ∈ R2, c ∈ R, then LINEAR(f) can be represented as an inner product: ∃ coefficient g ∈ R2 such that LINEAR(f) = g1 f1 + g2 f2 = gTf =: ⟨g, f⟩ ≡ g • f ∀f ∈ R2. Power Tool Generalization—Riesz Representation Theorem—gives error bounds for numerical algorithms, e.g., LINEAR(f) [0,1]d f(t) dt − 1 n n i=1 f(xi ) ⩽ BAD(x1 , . . . , xn ) BAD(f) 2/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Main Idea What seems obvious for two-dimensional vectors becomes a power tool for numerical analysis Obvious If LINEAR : R2 → R is any linear, real-valued function, meaning, LINEAR(cf + h) = c LINEAR(f) + LINEAR(h) ∀f, h ∈ R2, c ∈ R, then LINEAR(f) can be represented as an inner product: ∃ coefficient g ∈ R2 such that LINEAR(f) = g1 f1 + g2 f2 = gTf =: ⟨g, f⟩ ≡ g • f ∀f ∈ R2. Power Tool Generalization—Riesz Representation Theorem—gives error bounds for numerical algorithms, e.g., LINEAR(f) [0,1]d f(t) dt average of f e.g., option price − 1 n n i=1 f(xi ) average of f values e.g., payoffs under various scenarios ⩽ BAD(x1 , . . . , xn ) BAD(f) 2/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Main Idea What seems obvious for two-dimensional vectors becomes a power tool for numerical analysis Obvious If LINEAR : R2 → R is any linear, real-valued function, then LINEAR(f) can be represented as an inner product Power Tool Generalization—Riesz Representation Theorem—gives error bounds for numerical algorithms, e.g., LINEAR(f) [0,1]d f(t) dt average of f e.g., option price − 1 n n i=1 f(xi ) average of f values e.g., payoffs under various scenarios ⩽ BAD(x1 , . . . , xn ) Concentrate on choosing the xi well BAD(f) 2/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Why Is this My Favorite Theorem? [0,1]d f(t) dt − 1 n n i=1 f(xi ) ⩽ BAD(x1 , . . . , xn ) BAD(f), My most cited paper [1] according to Google Scholar is a simple application of the Riesz Representation Theorem 3/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for R2 Theorem If LINEAR : R2 → R is any linear, real-valued function, and ⟨h, f⟩ := h1 f1 + h2 f2 = hTf, then there exists a unique representer g ∈ R2, dependent on LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ R2. Proof. Existence. Let e1 = (1, 0)T and e2 = (0, 1)T. Then for all f = (f1 , f2 )T, LINEAR(f) = LINEAR e1 f1 + e2 f2 = LINEAR(e1 )f1 + LINEAR(e2 )f2 by linearity = ⟨g, f⟩ , where g = LINEAR(e1 ), LINEAR(e2 ) T 4/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for R2 Theorem If LINEAR : R2 → R is any linear, real-valued function, and ⟨h, f⟩ := h1 f1 + h2 f2 = hTf, then there exists a unique representer g ∈ R2, dependent on LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ R2. Proof. Existence. Let e1 = (1, 0)T and e2 = (0, 1)T. Then for all f = (f1 , f2 )T, LINEAR(f) = LINEAR e1 f1 + e2 f2 = LINEAR(e1 )f1 + LINEAR(e2 )f2 by linearity = ⟨g, f⟩ , where g = LINEAR(e1 ), LINEAR(e2 ) T Example. If LINEAR(e1 ) = −3 and LINEAR(e2 ) = 2, then LINEAR(f) = −3f1 , +2f2 = (−3, 2)T, f . 4/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for R2 Theorem If LINEAR : R2 → R is any linear, real-valued function, and ⟨h, f⟩ := h1 f1 + h2 f2 = hTf, then there exists a unique representer g ∈ R2, dependent on LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ R2. Proof. Existence. Done. Uniqueness. If g and ˜ g are both representers, i.e., LINEAR(f) = ⟨g, f⟩ = ⟨ ˜ g, f⟩ for all f ∈ Rd, then 0 = LINEAR(g − ˜ g) − LINEAR(g − ˜ g) = ⟨g, g − ˜ g⟩ − ⟨ ˜ g, g − ˜ g⟩ = ⟨g − ˜ g, g − ˜ g⟩ = ∥g − ˜ g∥2 so g − ˜ g = 0 and g = ˜ g. 4/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes The Dot/Inner Product My experience with f • h ≡ ⟨f, h⟩ Geometry (Euclidean, crow flying) distance/size/length/norm: ∥f∥ := f2 1 + f2 2 Pythagorean Theorem Trigonometry Law of cosines: ∥f − h∥2 = ∥f∥2 + ∥h∥2 − 2 ∥f∥ ∥h∥ cos(∡(f, h)) Physics f • h := ∥f∥ ∥h∥ cos(∡(f, h)) f • h = f1 h1 + f2 h2 5/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes The Dot/Inner Product My experience with f • h ≡ ⟨f, h⟩ Geometry (Euclidean, crow flying) distance/size/length/norm: ∥f∥ := f2 1 + f2 2 Pythagorean Theorem Trigonometry Law of cosines: ∥f − h∥2 = ∥f∥2 + ∥h∥2 − 2 ∥f∥ ∥h∥ cos(∡(f, h)) Physics f • h := ∥f∥ ∥h∥ cos(∡(f, h)) f • h = f1 h1 + f2 h2 Right way to think about ⟨f, h⟩ for arbitrary vectors spaces V, e.g., spaces of functions. Inner product first: ⟨·, ·⟩ : V × V → R satisfying ⟨0, 0⟩ = 0, ⟨f, f⟩ > 0 ∀f ̸= 0, ⟨f, h⟩ = ⟨h, f⟩ ⟨cf + h, k⟩ = c ⟨f, k⟩ + c ⟨h, k⟩ Then distance and cosine ∥f∥ := ⟨f, f⟩ cos(∡(f, h)) := ⟨f, h⟩ ∥f∥ ∥h∥ ∈ [−1, 1] by Cauchy-Schwartz Proof Law of cosines follows 5/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Hilbert space = a vector space (vectors can be added and multiplied by scalars) that is complete under ∥·∥ (sequences that should converge do) e.g., R is complete, Q is not may be infinite dimensional, e.g., all f : [0, 1] → R with a first derivative bounded means sup f∈V |LINEAR(f)| ∥f∥ < ∞ (automatic for finite dimensional V, but look here ) linear + bounded =⇒ continuous, linear + continuous =⇒ bounded Can we prove this theorem without referring to a basis for V? 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Proof. Existence. Define ker(LINEAR) = {v ∈ V : LINEAR(v) = 0} as the subspace of V that LINEAR maps into 0. If ker(LINEAR) = V, then all vectors in V are mapped to 0 and g = 0. 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Proof. Existence. Define ker(LINEAR) = {v ∈ V : LINEAR(v) = 0} as the subspace of V that LINEAR maps into 0. If ker(LINEAR) = V, then all vectors in V are mapped to 0 and g = 0. Otherwise, pick any nonzero g⊥ ∈ {u ∈ V : ⟨u, v⟩ = 0 ∀v ∈ ker(LINEAR)} How? , i.e., g⊥ is orthogonal to all vectors in ker(LINEAR). 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Proof. Existence. Define ker(LINEAR) = {v ∈ V : LINEAR(v) = 0} as the subspace of V that LINEAR maps into 0. If ker(LINEAR) = V, then all vectors in V are mapped to 0 and g = 0. Otherwise, pick any nonzero g⊥ ∈ {u ∈ V : ⟨u, v⟩ = 0 ∀v ∈ ker(LINEAR)} How? , i.e., g⊥ is orthogonal to all vectors in ker(LINEAR). For any f ∈ V, let h = LINEAR(f)g⊥ − LINEAR(g⊥ )f, and note that LINEAR(h) = LINEAR LINEAR(f)g⊥ − LINEAR(g⊥ )f = LINEAR(f) LINEAR(g⊥ ) − LINEAR(g⊥ ) LINEAR(f) = 0, so h ∈ ker(LINEAR). 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Proof. Existence. Define ker(LINEAR) = {v ∈ V : LINEAR(v) = 0} as the subspace of V that LINEAR maps into 0. If ker(LINEAR) = V, then all vectors in V are mapped to 0 and g = 0. Otherwise, pick any nonzero g⊥ ∈ {u ∈ V : ⟨u, v⟩ = 0 ∀v ∈ ker(LINEAR)} How? , i.e., g⊥ is orthogonal to all vectors in ker(LINEAR). For any f ∈ V, let h = LINEAR(f)g⊥ − LINEAR(g⊥ )f, and note that LINEAR(h) = LINEAR LINEAR(f)g⊥ − LINEAR(g⊥ )f = LINEAR(f) LINEAR(g⊥ ) − LINEAR(g⊥ ) LINEAR(f) = 0, so h ∈ ker(LINEAR). The choice of g⊥ implies that 0 = ⟨g⊥ , h⟩ = LINEAR(f) ⟨g⊥ , g⊥ ⟩ − LINEAR(g⊥ ) ⟨g⊥ , f⟩ , LINEAR(f) = LINEAR(g⊥ ) ⟨g⊥ , f⟩ ⟨g⊥ , g⊥ ⟩ = ⟨g, f⟩ for g := LINEAR(g⊥ )g⊥ ∥g∥2 . 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Riesz Representation Theorem for a Hilbert Space (V, ⟨·, ·⟩) Theorem (Riesz Representation Theorem for Hilbert Spaces) If LINEAR : V → R is any bounded linear real-valued function on the Hilbert space (V, ⟨·, ·⟩), then there exists a unique g ∈ V, called the representer of LINEAR, for which LINEAR(f) = ⟨g, f⟩ for all f ∈ V. Proof. Existence. Done. Uniqueness. Same proof as before. 6/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes What Can We Do with the Riesz Representation Theorem? Theorem (Error Bound for Numerical Integration) Let (V, ⟨·, ·⟩) be a Hilbert space of functions on [0, 1]d. Suppose that integration and function evaluation are both bounded, linear real-valued functions on V. Then the error of approximating the integral of a function in V by the sample mean is [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ = cos ∡(η, f) ∥η∥ ∥f∥ ⩽ ∥η∥ ∥f∥ ∀f ∈ V, for some representer η ∈ V that depends on x1 , . . . , xn , but not on f. Significance Error bound separates the f dependent part from the algorithm dependent part (η) Algorithm developers can concentrate on making ∥η∥ small Providential if η is nearly orthogonal to f [2], but don’t count on it 7/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes What Can We Do with the Riesz Representation Theorem? Theorem (Error Bound for Numerical Integration) Let (V, ⟨·, ·⟩) be a Hilbert space of functions on [0, 1]d. Suppose that integration and function evaluation are both bounded, linear real-valued functions on V. Then the error of approximating the integral of a function in V by the sample mean is [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ = cos ∡(η, f) ∥η∥ ∥f∥ ⩽ ∥η∥ ∥f∥ ∀f ∈ V, for some representer η ∈ V that depends on x1 , . . . , xn , but not on f. Proof. Note that INT : f → [0,1]d f(t) dt and AVG : f → 1 n n i=1 f(xi ) are bounded, linear real-valued functions. 7/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes What Can We Do with the Riesz Representation Theorem? Theorem (Error Bound for Numerical Integration) Let (V, ⟨·, ·⟩) be a Hilbert space of functions on [0, 1]d. Suppose that integration and function evaluation are both bounded, linear real-valued functions on V. Then the error of approximating the integral of a function in V by the sample mean is [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ = cos ∡(η, f) ∥η∥ ∥f∥ ⩽ ∥η∥ ∥f∥ ∀f ∈ V, for some representer η ∈ V that depends on x1 , . . . , xn , but not on f. Proof. Note that INT : f → [0,1]d f(t) dt and AVG : f → 1 n n i=1 f(xi ) are bounded, linear real-valued functions. Thus, so is ERR = INT − AVG. By the Reisz Representation Theorem, there exists a representer η ∈ V such that [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ERR(f) = ⟨η, f⟩. 7/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes What Can We Do with the Riesz Representation Theorem? Theorem (Preliminary Error Bound for Numerical Integration) Let (V, ⟨·, ·⟩) be a Hilbert space of functions on [0, 1]d. Suppose that integration and function evaluation are both bounded, linear real-valued functions on V. Then the error of approximating the integral of a function in V by the sample mean is [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ = cos ∡(η, f) ∥η∥ ∥f∥ ⩽ ∥η∥ ∥f∥ ∀f ∈ V, for some representer η ∈ V that depends on x1 , . . . , xn , but not on f. Significance Error bound separates the f dependent part from the algorithm dependent part (η) Algorithm developers can concentrate on making ∥η∥ small Providential if η is nearly orthogonal to f [2], but don’t count on it How to find η? 7/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Reproducing Kernels [3] Suppose that (V, ⟨·, ·⟩) is Hilbert space of functions on Ω for which function evaluation is a bounded, linear functional. Then there exists, K : Ω × Ω → R called a reproducing kernel for which K(t, x) = K(x, t) symmetry , K(·, x) ∈ V belonging , f(x) = ⟨K(·, x), f⟩ reproduction ∀t, x ∈ Ω, f ∈ V What do reproducing kernels look like for V = Rd Look here 8/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Reproducing Kernels [3] Suppose that (V, ⟨·, ·⟩) is Hilbert space of functions on Ω for which function evaluation is a bounded, linear functional. Then there exists, K : Ω × Ω → R called a reproducing kernel for which K(t, x) = K(x, t) symmetry , K(·, x) ∈ V belonging , f(x) = ⟨K(·, x), f⟩ reproduction ∀t, x ∈ Ω, f ∈ V Combining with the Riesz Representation Theorem ERR(f) := [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ , representer η =? η(x) = reproduction ⟨K(·, x), η⟩ = symmetry ⟨η, K(·, x)⟩ = representer ERR K(·, x) 8/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Reproducing Kernels [3] Suppose that (V, ⟨·, ·⟩) is Hilbert space of functions on Ω for which function evaluation is a bounded, linear functional. Then there exists, K : Ω × Ω → R called a reproducing kernel for which K(t, x) = K(x, t) symmetry , K(·, x) ∈ V belonging , f(x) = ⟨K(·, x), f⟩ reproduction ∀t, x ∈ Ω, f ∈ V Combining with the Riesz Representation Theorem ERR(f) := [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ , representer η =? η(x) = reproduction ⟨K(·, x), η⟩ = symmetry ⟨η, K(·, x)⟩ = representer ERR K(·, x) = [0,1]d K(t, x) dt − 1 n n i=1 K(xi , x) 8/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Reproducing Kernels [3] Suppose that (V, ⟨·, ·⟩) is Hilbert space of functions on Ω for which function evaluation is a bounded, linear functional. Then there exists, K : Ω × Ω → R called a reproducing kernel for which K(t, x) = K(x, t) symmetry , K(·, x) ∈ V belonging , f(x) = ⟨K(·, x), f⟩ reproduction ∀t, x ∈ Ω, f ∈ V Combining with the Riesz Representation Theorem ERR(f) := [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ , representer η =? η(x) = reproduction ⟨K(·, x), η⟩ = symmetry ⟨η, K(·, x)⟩ = representer ERR K(·, x) = [0,1]d K(t, x) dt − 1 n n i=1 K(xi , x) ∥η∥2 = ⟨η, η⟩ = representer ERR(η) 8/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Reproducing Kernels [3] Suppose that (V, ⟨·, ·⟩) is Hilbert space of functions on Ω for which function evaluation is a bounded, linear functional. Then there exists, K : Ω × Ω → R called a reproducing kernel for which K(t, x) = K(x, t) symmetry , K(·, x) ∈ V belonging , f(x) = ⟨K(·, x), f⟩ reproduction ∀t, x ∈ Ω, f ∈ V Combining with the Riesz Representation Theorem ERR(f) := [0,1]d f(t) dt − 1 n n i=1 f(xi ) = ⟨η, f⟩ , representer η =? η(x) = reproduction ⟨K(·, x), η⟩ = symmetry ⟨η, K(·, x)⟩ = representer ERR K(·, x) = [0,1]d K(t, x) dt − 1 n n i=1 K(xi , x) ∥η∥2 = ⟨η, η⟩ = representer ERR(η) = [0,1]2d K(t, x) dt dx − 2 n n i=1 [0,1]d K(xi , x) dx + 1 n2 n i,j=1 K(xi , xj ) 8/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Putting It Together Theorem (Error Bound for Numerical Integration) Let (V, ⟨·, ·⟩) be a Hilbert space of functions on [0, 1]d with reproducing kernel, K. Suppose that integration and function evaluation are both bounded, linear real-valued functions on V. Then the error of approximating the integral of a function in V by the sample mean is [0,1]d f(t) dx − 1 n n i=1 f(xi ) ⩽ BAD(x1 , . . . , xn ) BAD(f), where BAD2(x1 , . . . , xn ) = [0,1]2d K(t, x) dt dx − 2 n n i=1 [0,1]d K(xi , x) dx + 1 n2 n i,j=1 K(xi , xj ), BAD(f) = ∥f∥ . For an explicit example of a K and BAD(x1 , . . . , xn ) Look here 9/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Why Do Others Cite This Paper? [0,1]d f(t) dt − 1 n n i=1 f(xi ) ⩽ BAD(x1 , . . . , xn ) BAD(f), You can pick reproducing kernel K well and analyze Convergence How fast BAD(x1 , . . . , xn ) → 0 with n for clever x1 , x2 , . . . Tractability How this convergence depends on d 10/17

Thank you These slides are available at speakerdeck.com/riesz-representation-theorem

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes References 1. H., F. J. A Generalized Discrepancy and Quadrature Error Bound. Math. Comp. 67, 299–322 (1998). 2. H., F. J. The Trio Identity for Quasi-Monte Carlo Error Analysis. in Monte Carlo and Quasi-Monte Carlo Methods: MCQMC, Stanford, USA, August 2016 (eds Glynn, P. & Owen, A.) (Springer-Verlag, Berlin, 2018), 3–27. 3. N. Aronszajn. Theory of Reproducing Kernels. Trans. Amer. Math. Soc. 68, 337–404 (1950). 12/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes How Can a Linear Function on a Hilbert Space be Unbounded? Back Short answer: V must be infinite dimensional 13/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes How Can a Linear Function on a Hilbert Space be Unbounded? Back Short answer: V must be infinite dimensional Consider a vector space of real-valued sequences with the typical inner product: V := {f = (f1 , f2 , . . .)T : fi ∈ R, ∥f∥ < ∞}, ⟨f, h⟩ := f1 h1 + f2 h2 + f3 h3 + · · · Define the linear real-valued function LINEAR(f) = f1 + 2f2 + 3f3 + 4f4 + · · · Let ei := (0, . . . , 0, 1 ith position , 0, . . .)T. For any ε, δ > 0, choose i > ε/δ. Then ∥δei∥ = δ, but sup f̸=0 |LINEAR(f)| ∥f∥ ⩾ sup i=1,2,... |LINEAR(δei )| ∥δei∥ = sup i=1,2,... iδ δ = sup i=1,2,... i = ∞ Cannot guarantee that |LINEAR(f)| is small enough, no matter how small you make ∥f∥. This LINEAR is unbounded. 13/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Proof of the Cauchy-Schwarz Inequality Back Theorem (Cauchy-Schwarz) Let (V, ⟨·, ·⟩) be a real-valued inner product space. Then ⟨f, h⟩ ⩽ ∥f∥ ∥h∥ ∀f, h ∈ V, (C-S) with equality iff c1 f + c2 h = 0 for some nonzero (c1 , c2 ). Proof of Inequality. If f or h are zero, the inequality becomes an equality by direct calculation. For any nonzero f, h ∈ V, define the quadratic polynomial p as follows: p(t) := ∥tf + h∥2 = ⟨tf + h, tf + h⟩ = t2 ⟨f, f⟩ + 2t ⟨f, h⟩ + ⟨h, h⟩ = t2 ∥f∥2 + 2t ⟨f, h⟩ + ∥h∥2 . Since p(t) ⩾ 0 by definition, p cannot have two roots, which means that ⟨f, h⟩ 2 − ∥f∥2 ∥h∥2 must be non-positive. This implies the inequality for ⟨f, h⟩ . 14/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes Proof of the Cauchy-Schwarz Inequality Back Theorem (Cauchy-Schwarz) Let (V, ⟨·, ·⟩) be a real-valued inner product space. Then ⟨f, h⟩ ⩽ ∥f∥ ∥h∥ ∀f, h ∈ V, (C-S) with equality iff c1 f + c2 h = 0 for some nonzero (c1 , c2 ). Proof of Equality. Recall that p(t) := ∥tf + h∥2 = t2 ∥f∥2 + 2t ⟨f, h⟩ + ∥h∥2 . Equality in (C-S) happens iff ⟨f, h⟩ = ∥f∥ ∥h∥, which implies that p has a single root, t0 . Then 0 = p(t0 ) = ∥t0 f + h∥2 , which is true iff t0 f + h = 0 by the definition of a norm. 14/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes There Exists a Nonzero g⊥ Orthogonal to All of ker(V) Back Lemma If LINEAR : V → R is any bounded, linear functional on the Hilbert space (V, ⟨·, ·⟩), and ker(LINEAR) := LINEAR−1({0}) = {f ∈ V : LINEAR(f) = 0} ̸= V, then there exists a nonzero g⊥ ∈ {h ∈ V : ⟨h, f⟩ = 0 ∀f ∈ ker(LINEAR)}. Proof. Define dist h, ker(LINEAR) := inf{∥h − f∥ : f ∈ ker(LINEAR)}, i.e., the closest ker(LINEAR) comes to h. For any h / ∈ ker(LINEAR), choose kn ∈ ker(LINEAR) such that ∥h − kn∥2 ⩽ dist2 h, ker(LINEAR) + 1/n for n = 1, 2, . . .. By the parallelogram law, ∥km − kn∥2 = ∥(km − h) − (kn − h)∥2 = 2 ∥km − h∥2 + 2 ∥kn − h∥2 − ∥(km − h) + (kn − h)∥2 = 2 ∥km − h∥2 + 2 ∥kn − h∥2 − 4 ∥(km + kn )/2 − h)∥2 (km + kn )/2 ∈ ker(LINEAR)) ⩽ 2 dist2 h, ker(LINEAR) + 1/m] + 2 dist2 h, ker(LINEAR) + 1/n] − 4 dist2 h, ker(LINEAR) = 2(1/m + 1/n) So, kn → k ∈ V due to completeness of V; k ∈ ker(LINEAR) due to continuity of LINEAR. ∥h − k∥ = lim ∥h − kn∥ = lim dist2 h, ker(LINEAR) + 1/n = dist h, ker(LINEAR) 15/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes There Exists a Nonzero g⊥ Orthogonal to All of ker(V) Back Lemma If LINEAR : V → R is any bounded, linear functional on the Hilbert space (V, ⟨·, ·⟩), and ker(LINEAR) := LINEAR−1({0}) = {f ∈ V : LINEAR(f) = 0} ̸= V, then there exists a nonzero g⊥ ∈ {h ∈ V : ⟨h, f⟩ = 0 ∀f ∈ ker(LINEAR)}. Proof. Let g⊥ = h − k. Since h / ∈ ker(LINEAR) and k ∈ ker(LINEAR), then h ̸= k and ∥g⊥ ∥ = ∥h − k∥ = dist h, ker(LINEAR) ̸= 0. Pick any f ∈ ker(LINEAR), and note that k + tf ∈ ker(LINEAR) for all t ∈ R. Thus, ∥g⊥ ∥2 ⩽ ∥h − (k + tf)∥2 = ∥g⊥ − tf∥2 = ∥g⊥ ∥2 − 2t ⟨g⊥ , f⟩ + t2 ∥f∥2 0 ⩽ t −2 ⟨g⊥ , f⟩ + t ∥f∥2 ∀t ∈ R The only way to ensure this inequality for all t is for ⟨g⊥ , f⟩ = 0. 15/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes What Does a Reproducing Kernel Look Like for V = Rd? Back The functions have domain Ω := {1, . . . , d}, and are represented as vectors of matrices. Pick a symmetric, positive definite (all eigenvalues are positive) matrix A ∈ Rd×d to define the inner product ⟨f, h⟩ := fTAh, where f = f(i) d i=1 . Then the reproducing kernel, K, is defined by K(i, j) d i,j=1 = K := A−1. Note that K(i, j) = K(j, i) because A is symmetric and thus so is K K(·, j) = jth column of K =: Kj ∈ Rd = V ⟨K(·, j), f⟩ = KT j Af = ej f = f(j) since K := A−1 where ej := (0, . . . , 0, 1 jth position , 0, . . .)T 16/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes An Example of the Cubature Error Bound [1] Back Consider the reproducing kernel K(t, x) := d k=1 1 + 1 2 |tk − 1/2| + |xk − 1/2| − |tk − xk | t, x ∈ [0, 1]d Then BAD2(x1 , . . . , xn ) = 13 12 d − 2 n n i=1 d k=1 1 + 1 2 |xik − 1/2| − |xik − 1/2|2 + 1 n2 n i,j=1 d k=1 1 + 1 2 |xik − 1/2| + xjk − 1/2 − xik − xjk Requires O(dn2) operations to compute 17/17

Background Riesz Rep Thm in R2 Inner Product Riesz Rep Thm for Hilbert Spaces Reproducing Kernels References Deleted Scenes An Example of the Cubature Error Bound [1] Back Consider the reproducing kernel K(t, x) := d k=1 1 + 1 2 |tk − 1/2| + |xk − 1/2| − |tk − xk | t, x ∈ [0, 1]d which corresponds to the Hilbert space for functions defined on [0, 1]d with the following norm: ∥f∥2 := |f(1/2, . . . , 1/2)|2 + [0,1] ∂f ∂x1 (x1 , 1/2, . . . , 1/2) 2 dx1 + [0,1] ∂f ∂x1 (1/2, x2 , 1/2, . . . , 1/2) 2 dx2 + · · · + [0,1]2 ∂2f ∂x1 ∂x2 (x1 , x2 , 1/2, . . . , 1/2) 2 dx1 dx2 + · · · + [0,1]d ∂df ∂x1 · · · ∂xd (x) 2 dx BAD(f) = ∥f − f(1/2, . . . , 1/2)∥ 17/17