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'''2.2 Existence Theorem''' | '''2.2 Existence Theorem''' | ||
| − | <font size="3px"> First we are going to define the '''continuity''' of a function. Similar with what we have learnt in Calculus 1 but replaced by a 2-variable function, <math>f(t,y)</math> is continuous at the point <math>(t_0, y_0)</math> if here, it is defined, its limit exists and the function value equals to the limit value. | + | <font size="3px"> First we are going to define the '''continuity''' of a function. Similar with what we have learnt in Calculus 1 but replaced by a 2-variable function, <math>f(t,y)</math> is continuous at the point <math>(t=t_0,y=y_0)</math> if here, it is defined, its limit exists and the function value equals to the limit value. |
| − | Now let's make an extension. Imagine we are looking at part of a graph of a function by a magnifier, then suppose <math>f(t,y)</math> is continuous in a closed bounded area (seeing the graph from above the magnifier) in coordinates. So <math>f(t,y)</math> will be continuous on <math>R</math> if it's continuous at each point in <math>R</math>. To understand this, just imagine we are smoothly moving our magnifier over the graph, and we can see the graph is always continuous, without any exception. Hence, we know there exist solution(s) to this ODE.</font> | + | Now let's make an extension. Imagine we are looking at part of a graph of a function by a magnifier, then suppose <math>f(t,y)</math> is continuous in a closed bounded area (seeing the graph from above the magnifier) in coordinates. So <math>f(t,y)</math> will be continuous on <math>R^2</math> if it's continuous at each point in <math>R^2</math>. To understand this, just imagine we are smoothly moving our magnifier over the graph, and we can see the graph is always continuous, without any exception. |
| + | |||
| + | The geometric meaning of a function with one variable is a line or curve. We know a dot moves to form a line, and a line moves to form a surface. So the geometric meaning of a function with two variables is going to be a surface in three-dimensional coordinates. Since <math>f(t,y)</math> is continuous at every point in <math>R^3</math>, it will always touch the coordinates, geometrically. Hence, we know there exist solution(s) to this ODE.</font> | ||
Revision as of 17:49, 16 October 2017
The Existence and Uniqueness Theorem for Solutions to ODEs
2.0 Abstract
Before starting this tutorial, you are supposed to be able to:
· Find an explicit solution for $ \frac{dy}{dt}=f(t) $. This is the same thing as finding the integral of $ f(t) $ with respect to $ t $.
· Know the difference between a general solution and a particular solution satisfying the initial conditions.
· Check one function is a solution to an ODE.
· Distinguish ODE and PDE, know the usual notations.
· Know the basic concepts of ODEs (order, linearity, homogeneity, etc).
2.1 Concept
From the first example from 1.1, here we still suppose that we had a linear equation $ ax+b=0 $ with respect to $ x $.
· When $ a=0 $, $ b≠0 $, there is no solution.
· When $ a≠0 $, there is one solution $ x=-\frac{b}{a} $.
· When $ a=b=0 $, there are infinitely many solutions to this linear equation.
Similarly, an ODE may also have no solution, a unique solution or infinitely many solutions. The existence theorem is used to check whether there exists a solution for an ODE, while the uniqueness theorem is used to check whether there is one solution or infinitely many solutions.
2.2 Existence Theorem
First we are going to define the continuity of a function. Similar with what we have learnt in Calculus 1 but replaced by a 2-variable function, $ f(t,y) $ is continuous at the point $ (t=t_0,y=y_0) $ if here, it is defined, its limit exists and the function value equals to the limit value.
Now let's make an extension. Imagine we are looking at part of a graph of a function by a magnifier, then suppose $ f(t,y) $ is continuous in a closed bounded area (seeing the graph from above the magnifier) in coordinates. So $ f(t,y) $ will be continuous on $ R^2 $ if it's continuous at each point in $ R^2 $. To understand this, just imagine we are smoothly moving our magnifier over the graph, and we can see the graph is always continuous, without any exception.
The geometric meaning of a function with one variable is a line or curve. We know a dot moves to form a line, and a line moves to form a surface. So the geometric meaning of a function with two variables is going to be a surface in three-dimensional coordinates. Since $ f(t,y) $ is continuous at every point in $ R^3 $, it will always touch the coordinates, geometrically. Hence, we know there exist solution(s) to this ODE.
