# 2250-4 12:25 Lecture Record Week 8 F2010

Last Modified: October 25, 2010, 09:20 MDT.    Today: July 23, 2018, 03:57 MDT.

## 18, 20, 21 Oct: Michal and Laura

Exam 2 review.
``` Problem sessions on ch4 problems. JWB 335 Mon-Wed and Web 103 Thursday this week
Exam 2 review for problems 1,2,3. Problems 4,5 review in regular lecture next week.
How to construct solutions to 4.7-10,20,26.
Questions answered on Chapter 4 problems.
Survey of solution methods for 4.3, 4.4, 4.5 problems.
Illustration: How to do abstract independence arguments using vector
packages, without looking inside the packages.
Applications of the Sampling test and Wronskian test for functions.
How to use the pivot theorem to identify independent vectors from a list.
```

## 19 Oct:Basis and dimension. Intro to Linear Differential Equations.

```PARTIAL FRACTION THEORY.
Examples.
top=x-1, bottom=(x+1)(x^2+1)
top=x-1, bottom=(x+1)^2(x^2+1)^2
Maple assist with convert(top/bottom,parfrac,x);
PROBLEM 4.7-26.
How to solve y''+10y'=0 for general solution y=c1 + c2 exp(-10x)
Outline of the general theory used to solve linear differential equations.
Order of a DE and the dimension of the solution space.
Euler's theorem.
Finding solution atoms for a basis.
PROOFS. [slides]
rank(A)=rank(A^T). Theorem 3, section 4.5.
How to prove pivot theorem from frame sequence facts.
ALGEBRAIC INDEPENDENCE TESTS: mostly review
Rank test.
Determinant test.
Sampling test.
Application to x^2,exp(x)
Wronskian test.
Application to 1,x,x^2,x^3
Orthogonal vector test.
Example: (1,1,0),  (1,-1,0), ((0,0,1)
Pivot theorem.
Example: Find the independent columns in a matrix.
Example: Find the maximum number of independent vectors in a list.
FUNCTIONS.
How to represent functions as graphs and as infinitely long column
vectors. Rules for add and scalar multiply. Independence tests
using functions as the vectors.
BASIS.
Definition of basis and span.
Examples: Find a basis from a general solution formula.
Bases and the pivot theorem.
Example: Find a basis for the row vectors in a matrix.
Example: Find a basis or the column vectors in a matrix.
Equivalence of bases.
Example: A subspace S contains vectors v1,v2 and also vectors w1,w2.
When are both v1,v2 and w1,w2 bases for S?
A computer test for equivalent bases.
DIMENSION.
THEOREM. Two bases for a vector space V must have the same number of vectors.

Examples:
Last Frame Algorithm: Basis for a linear system Ax=0.
Last frame algorithm and the vector general solution.
Basis of solutions to a homogeneous system of linear algebraic equations.
Bases and partial derivatives of the general solution on the invented symbols t1, t2, ...
DE Example: y = c1 e^x + c2 e^{-x} is the general solution. What's the basis?
SOLUTION ATOMS and INDEPENDENCE.
Def. atom=x^n(base atom)
base atom = 1, exp(ax), cos(bx), sin(bx), exp(ax) cos(bx), exp(ax) sin(bx)
"atom" abbreviates "solution atom of a linear differential equation"
THEOREM. Atoms are independent.
EXAMPLE. Show 1, x^2, x^9 are independent
EXAMPLE. Show 1, x^2, x^(3/2) are independent [Wronskian test]
PROBLEM 4.7-26. Review of Chapter 1 methods.
How to solve y''+10y'=0 for general solution y=c1 + c2 exp(-10x)
```

## 19 Oct: Introduction to higher order linear DE. Sections 5.1, 5.2.

```MAPLE LAB 2. [laptop projection]
Hand Solution to L2.2.
Graphic in L2.3.
Interpretation of graphics in L2.4.
Summary for Higher Order Differential Equations
Slides: Atoms, Euler's theorem, 7 examples (96.6 K, pdf, 20 Oct 2009)Slides: Base atom, atom, basis for linear DE (85.4 K, pdf, 20 Oct 2009)  EXAMPLE. The equation y'' +10y'=0. Review.
How to solve y'' + 10y' = 0 with chapter 1 methods. Midterm 1 problem 1(d).
Idea: Let v=x'(t) to get a first order DE in v and a quadrature equation x'(t)=v(t).
Solve the first order DE by the linear integrating factor method. Then insert
the answer into x'(t)=v(t) and continue to solve for x(t) by quadrature.
Vector space of functions: solution space of a differential equation.
A basis for the solution space of y'' + 10y'=0 is {1,exp(-10x)}
ATOMS.
Base atoms are 1, exp(a x), cos(b x), sin(b x), exp(ax)cos(bx), exp(ax)sin(bx).
DEFINITION: atom=x^n(base atom).
THEOREM. Atoms are independent.
THEOREM. Solutions of constant-coefficient homogeneous differential
equations are linear combinations of atoms.
PICARD'S THEOREM.  It says that nth order equations have a solution space of dimension n.
EULER'S THEOREM. It says y=exp(rx) is a solution of ay''+by'+cy=0 <==> r is
a root of the characteristic equation ar^2+br+c=0.
Shortcut: The characteristic equation can be synthetically formed from the
differential equation ay''+by'+cy=0 by the formal replacement
y ==> 1, y' ==> r, y'' ==> r^2.
EXAMPLE. The equation y''+10y'=0 has characteristic equation r^2+10r=0
with roots r=0, r=-10.
Then Euler's theorem says exp(0x) and exp(-10x) are solutions.
By vector space dimension theory, 1, exp(-10x) are a basis for
the solution space of the differential equation.
Then the general solution is
y = c1 (1) + c2 (exp(-10x)).

Survey of topics for this week.
Linear DE Slides.
Slides: Picard-Lindelof, linear nth order DE, superposition (121.7 K, pdf, 18 Oct 2009)  Slides: How to solve linear DE or any order (104.1 K, pdf, 18 Oct 2009)  Slides: Atoms, Euler's theorem, 7 examples (96.6 K, pdf, 20 Oct 2009)
Theory of Higher Order Constant Equations:
Homogeneous and non-homogeneous structure.
Superposition.
Picard's Theorem.
Solution space structure.
Dimension of the solution set.
Atoms.
Definition of atom.
Independence of atoms.
Euler's theorem.
Real roots
Non-real roots [complex roots].
How to deal with conjugate pairs of factors (r-a-ib), (r-a+ib).
The formula exp(i theta)=cos(theta) + i sin(theta).
How to solve homogeneous equations:
Use Euler's theorem to find a list of n distinct solution atoms.
Examples:   y''=0, y''+3y'+2y=0, y''+y'=0, y'''+y'=0.

Second order equations.
Homogeneous equation.
Harmonic oscillator example y'' + y=0.
Picard-Lindelof theorem.
Dimension of the solution space.
Structure of solutions.
Non-homogeneous equation. Forcing term.
Nth order equations.
Solution space theorem for linear differential equations.
Superposition.
Independence and Wronskians. Independence of atoms.
Main theorem on constant-coefficient equations [Solutions are linear combinations of atoms].
Euler's substitution y=exp(rx).
Shortcut to finding the characteristic equation.
Euler's basic theorem:
y=exp(rx) is a solution <==> r is a root of the characteristic equation.
Euler's multiplicity theorem:
y=x^n exp(rx) is a solution <==> r is a root of multiplicity n+1 of the characteristic equation.
How to solve any constant-coefficient homogeneous differential equation.
Picard's Theorem for higher order DE and systems.

21 Oct: Constant coefficient equations with complex roots

PROBLEM SESSION.
Chapter 4 exercises.
Constant coefficient equations with complex roots.
How to solve for atoms when the characteristic equation has multiple roots or complex roots.
Applying Euler's theorems to solve a DE.
Examples of order 2,3,4. Exercises 5.1, 5.2, 5.3.
Applications.
Spring-mass system,
RLC circuit equation.
harmonic oscillation,

21 Oct: Second order and higher order differential Equations

Second order and higher order differential Equations.
Picard theorem for second order equations, superposition, solution
space structure, dimension of the solution set.
Euler's theorem.
Constant-coefficient second order homogeneous differential equations.
Characteristic equation and its factors determine the atoms.
Sample equations:
y''=0, y''+2y'+y=0, y''-4y'+4y=0,
y''+y=0, y''+3y'+2y=0,
mx''+cx'+kx=0, LQ''+RQ'+Q/C=0.
Solved examples like the 5.1,5.2,5.3 problems.
Solving a DE when the characteristic equation has complex roots.
Higher order equations or order 3 and 4.
Finding 2 atoms from one complex root.
Why the complex conjugate root identifies the same two atoms.
Equations with both real roots and complex roots.
An equation with 4 complex roots. How to find the 4 atoms.
Review and Drill.
Solving more complicated homogeneous equations.
Example: Linear DE given by roots of the characteristic equation.
Example: Linear DE given by factors of the characteristic polynomial.
Example: Construct a linear DE of order 2 from a list of two atoms that must be solutions.
Example: Construct a linear DE from roots of the characteristic equation.
Example: Construct a linear DE from its general solution.

References:  Sections 5.4, 5.6. Forced oscillations.
Slides: Unforced vibrations 2008 (620.4 K, pdf, 11 Oct 2009) Slides: Forced undamped vibrations (174.7 K, pdf, 11 Oct 2009) Slides: Forced damped vibrations (235.0 K, pdf, 11 Oct 2009) Slides: Forced vibrations and resonance (185.3 K, pdf, 11 Oct 2009) Slides: Resonance and undetermined coefficients, cafe door, pet door, phase-amplitude (199.3 K, pdf, 20 Nov 2010) Slides: Electrical circuits (87.1 K, pdf, 11 Oct 2009)

```