MATH661

Lecture 33: Multiple Operators

1.Semi-discretization

In late nineteenth century, telegrapher's equations, a system of linear PDEs for current $I (x, t)$ and voltage $V (x, t)$

\frac{\partial}{\partial x} V (x, t) = - L \frac{\partial}{\partial t} I (x, t) - R I (x, t)

\frac{\partial}{\partial x} I (x, t) = - C \frac{\partial}{\partial t} C (x, t) - G V (x, t)

Heaviside avoided solution of the PDEs by reduction to an algebraic formulation historical formulation, e.g., for the ODE for $y (t)$

\frac{d y}{d t} + a y = b

Heaviside considered the associated algebraic problem for $Y (s)$

s Y + a Y = b \Rightarrow Y (s) = \frac{b}{a + s} \Rightarrow y (t) = ℒ^{- 1} [Y (s)]

Why should I refuse a good dinner simply because I don't understand the digestive processes involved? (Heaviside, ?)

Heaviside's formal framework (1890's) for solving ODEs was discounted since it lacked mathematical rigour.

Russian mathematician 1920's established first results (Vladimirov)
Theory of Distributions (Schwartz, 1950s)

2.Method of lines

Consider function $f : ℝ^{d} \to ℝ$ , $d ≫ 1$ assumed large, $f$ of unknown form, difficult to compute for general input. Seek $g : ℝ^{n} \to ℝ$ , $T : ℝ^{d} \to ℝ^{n}$ such that

|| f - g \circ T || < ε

for some $ε > 0$ .

Choose a basis set (Monomials, Exponentials, Wavelets) ${ϕ_{1}, ϕ_{2}, \dots}$ to approximation of $L^{2} (ℝ)$ functions in Hibert space

g_{n} (t) = \sum_{j = 1}^{n} (f, ϕ_{j}) ϕ_{j} = \sum_{j = 1}^{n} c_{j} ϕ_{j}

The approximation is convergent if

{lim}_{n \to \infty} || f - g \circ T || = 0,

This assumes $c_{j} =$ $(f, ϕ_{j})$ rapidly decrease.

Theorem. (Parseval) The Fourier transform is unitary. For $A, B : ℝ \to ℂ$ , square integrable, $2 π$ -periodic with Fourier series

$A (t) = \sum_{n = - \infty}^{\infty} a_{n} e^{i n t}, B (t) = \sum_{n = - \infty}^{\infty} b_{n} e^{i n t},$

$\sum_{n = - \infty}^{\infty} a_{n} {\overline{b}}_{n} = \frac{1}{2 π} \int_{- π}^{π} A (t) \overline{B} (t) d t .$

Bessel inequality:

\sum_{j = 1}^{n} {| (f, ϕ_{j}) |}^{2} ⩽ {|| f ||}_{2} .

Fourier coefficient decay: for $f \in C^{(k - 1)} (ℝ)$ , $f^{(k - 1)}$ absolutely continuous,

| c_{n} | ⩽ {min}_{0 ⩽ j ⩽ k} \frac{{|| f^{(j)} ||}_{1}}{{| n |}^{j}} .

In practice: coefficients decay as

$1 / n$ for functions with discontinuities on a set of Lebesgue measure 0;
$1 / n^{2}$ for functions with discontinuous first derivative on a set of Lebesgue measure 0;
$1 / n^{3}$ for functions with discontinuous second derivative on a set of Lebesgue measure 0.

Fourier coefficients for analytic functions decay faster than any monomial power $c_{n} = ο (n^{- p}), \forall p \in ℕ$ , a property known as exponential convergence.

Denote such approximations by $ℒ$ , and they are linear

ℒ (α f + β g) = α ℒ (f) + β ℒ (g)

Choose a basis set (Monomials, Exponentials, Wavelets) ${ϕ_{1}, ϕ_{2}, \dots}$ to approximation of $L^{2} (ℝ)$ functions in Hibert space

g_{n} (t) = \sum_{j = 1}^{n} c_{j} ϕ_{j}

Let $Φ_{n} = {φ_{k (1)}, φ_{k (2)}, \dots, φ_{k (n)}}$ such

(f, φ_{k (1)}) ⩾ (f, φ_{k (2)}) ⩾ \dots ⩾ (f, φ_{k (n)}) .

Choose $c_{j}$ = $(f, φ_{k (j)})$ , and

g_{n} (t) = \sum_{j = 1}^{n} c_{j} ϕ_{j} .

Denote such approximations by $𝒢$ , and they are non-linear.

3.Implicit-explicit methods

Consider function $f : ℝ^{d} \to ℝ$ , $d ≫ 1$ assumed large, $f$ of unknown form, difficult to compute for general input. Seek $g : ℝ^{n} \to ℝ$ , $T : ℝ^{d} \to ℝ^{n}$ such that

|| f - g \circ T || < ε

for some $ε > 0$ .

What questions do you ask?

Does $T exist ?$

$\forall f, ε, \exists T, such that || f - g \circ T || < ε$

Can arbitrary $ε$ be achieved?

Can we construct $T ?$

By what procedure?
$T = T_{1} \circ T_{2} \circ \dots \circ T_{J}$
with $T_{i}$ simple modifications of identity (ReLU)
${min}_{T_{1}, \dots T_{J}} || f - g \circ T_{1} \circ T_{2} \circ \dots \circ T_{J} ||$ $𝑻_{j} (𝒙) = η (𝑨_{j} 𝒙 + 𝒃_{j})$ $η (t) = {\begin{cases} 0 & t < 0 \\ t & t ⩾ 0 \end{cases} .$
At what cost?

How big is $n ?$