diff --git a/hw5/hw5.pdf b/hw5/hw5.pdf new file mode 100644 index 0000000..75d0a2a Binary files /dev/null and b/hw5/hw5.pdf differ diff --git a/hw5/hw5.tex b/hw5/hw5.tex new file mode 100644 index 0000000..080fece --- /dev/null +++ b/hw5/hw5.tex @@ -0,0 +1,75 @@ +% vim: set ts=2 sw=2 et tw=80: + +\documentclass[12pt,a4paper]{article} + +\usepackage[utf8]{inputenc} \usepackage[margin=2cm]{geometry} +\usepackage{amstext} \usepackage{amsmath} \usepackage{array} +\newcommand{\lra}{\Leftrightarrow} + +\title{Howework 5 -- Introduction to Computational Science} + +\author{Claudio Maggioni} + +\begin{document} \maketitle +\section*{Question 1} +Given the definition of degree of exactness being the highest polynomial degree +$n$ at which a quadrature, for every polynomial of degree $n$, produces exactly +the same polynomial, these are the proofs. + +\subsection*{Midpoint rule} +All polynomials of degree 1 can be expressed as: + +\[p_1(x) = a_1 \cdot x + a_0\] + +Therefore their integral is: + +\[\int_0^1 a_1 \cdot x + a_0 dx = \frac{a_1}{2} + a_0\] + +The midpoint rule for $p_1(x)$ is + +\[f\left(\frac{1}{2}\right) \cdot 1 = \frac{a_1}{2} + a_0 = \int_0^1 a_1 \cdot x ++ a_0 dx\] + +Therefore the midpoint rule has a degree of exactness of at least 1. + +It is easy to show that the degree of exactness is not higher than 1 by +considering the degree 2 +polynomial $x^2$, which has an integral in $[0, 1]$ of $\frac{1}{3}$ but a midpoint rule +quadrature of $\frac{1}{4}$. + +\subsection*{Trapezoidal rule} +The proof is similar to the one for the midpoint rule, but with this quadrature +for degree 1 polynomials: + +\[\frac{f(0)}{2} + \frac{f(1)}{2} = \frac{a_0 + a_1 + a_0}{2} = \frac{a_1}{2} + +a_0\] + +Which is again equal to the general integral for these polynomials. + +Again $x^2$ is a degree 2 polynomial with integral $\frac{1}{3}$ but a midpoint quadrature +of $\frac{0 + 1}{2} = \frac{1}{2}$, thus bounding the degree of exactness to 1. + +\subsection*{Simpson rule} +The proof is again similar, but for degree 3 polynomials which can all be +written as: + +\[p_3(x) = a_3 x^3 + a_2 x^2 + a_1 x + a_0\] + +The integral is: + +\[\int_0^1p_3(x) dx = \frac{a_3}{4} + \frac{a_2}{3} + \frac{a_1}{2} + a_0\] + +The Simpson rule gives: + +\[\frac{1}{6} \cdot f(0) + \frac{4}{6}\cdot f\left(\frac{1}{2}\right) + \frac{1}{6} \cdot f(1) = +\frac{1}{6} a_0 + \frac{4}{6} \left(\frac{a_3}{8} + \frac{a_2}{4} + +\frac{a_1}{2} + a_0\right) + \]\[\frac{1}{6} \left(a_3 + a_2 + a_1 + a_0\right) = +\frac{a_3}{4} + \frac{a_2}{3} + \frac{a_1}{2} + a_0 = \int_0^1 p_3(x)dx\] + +Which tells us that the degree of exactness is at least 1. + +We can bound the degree of exactness to 3 with the 4th degree polynomial $x^4$ +which has integral in $[0, 1]$ of $\frac{1}{5}$ but has a quadrature of +$\frac{1}{6} \cdot 0 + \frac{2}{3} \cdot \frac{1}{16} + \frac{1}{6} \cdot 1 = +\frac{5}{24}$. +\end{document}