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(Elementary and (Not So Elementary) Errors in Homework)
(Elementary and (Not So Elementary) Errors in Homework)
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(2) '''Algorithm vs. Proof'''
 
(2) '''Algorithm vs. Proof'''
  
When solving a problem that requires a solution to a linear equation, it is not always obvious which of the following you should show:
+
When solving a problem that requires a solution to a linear equation, it is not always obvious which of the following you
 +
 
 +
should show:
 
   
 
   
 
:a) An algorithm for finding the solution
 
:a) An algorithm for finding the solution

Revision as of 20:59, 15 October 2014

Contents

Boris

Elementary and (Not So Elementary) Errors in Homework

(1) Bad Notation


Let  
M_1 =
  \begin{pmatrix}
   1 & 0 \\
   0 & 0
  \end{pmatrix}, 
M_2 =
   \begin{pmatrix}
   0 & 0 \\
   0 & 1 \\
   \end{pmatrix}, 
M_3 =
   \begin{pmatrix}
   0 & 1 \\
   1 & 0 \\
   \end{pmatrix}
be matrices.


We want to equate span(M_1, M_2, M_3) to the set of all symmetric 2 \times 2 matrices. Here is the wrong way to write this:


Let 
span(M_1, M_2, M_3) =
  \begin{pmatrix}
   a & b \\
   b & c \\
  \end{pmatrix}
.


Firstly, span(M_1, M_2, M_2) is the set of all linear combinations of M_1, M_2, M_3. To equate it to a single

symmetric 2 \times 2 matrix makes no sense. Secondly, the elements a, b, c, d are undefined. What are they suppose to

represent? Rational numbers? Real numbers? Members of the field of two elements? The following way of writing erases those issues:


Let 
span(M_1, M_2, M_3) = \{
  \begin{pmatrix}
   a & b \\
   b & c \\
  \end{pmatrix}
  :a, b, c \in F \}
where F is an arbitrary field.


(2) Algorithm vs. Proof

When solving a problem that requires a solution to a linear equation, it is not always obvious which of the following you

should show:

a) An algorithm for finding the solution
b) A proof that a solution is correct

If the problem asks to solve a linear equation, then just show (a). Otherwise, for problems such as this:

Determine if the vector (-2, 2, 2) is a linear combination of the vectors (- (1, 2, -1), (-3, -3, 3) in R^3.

Show both (a) and (b) to be on the safe side.

Problem 5h) in Homework 3 for all Fields

For an arbitrary field F, determine if the matrix 
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
is in span S=\{
  \begin{pmatrix}
   1 & 0 \\
   -1 & 0 \\
  \end{pmatrix},
  \begin{pmatrix}
   0 & 1 \\
   0 & 1 \\
  \end{pmatrix},
  \begin{pmatrix}
   1 & 1 \\
   0 & 0 \\
  \end{pmatrix}
\}
.


Proof:

We show that 
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\in span(S) \iff char(F)=2.

We show that char(F)=2 \implies
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\in span(S)
.
Assume that char(F)=2.
Let c_1=0, c_2=1, c_3=1.
Then c_1, c_2, c_3 \in F.
Then 
c_1
  \begin{pmatrix}
   1 & 0 \\
   -1 & 0 \\
  \end{pmatrix}
+ c_2
  \begin{pmatrix}
   0 & 1 \\
   0 & 1 \\
  \end{pmatrix}
+ c_3
  \begin{pmatrix}
   1 & 1 \\
   0 & 0 \\
  \end{pmatrix}
=
  \begin{pmatrix}
   1 & 2 \\
   0 & 1 \\
  \end{pmatrix}
.
Since char(F)=2 and the entries of the matrix are from F, then 0=2.
Then 
  \begin{pmatrix}
   1 & 2 \\
   0 & 1 \\
  \end{pmatrix}
=
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\in span(S)
.
Then 
char(F)=2 \implies
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\in span(S)
.
We show that char(F) \neq 2 \implies
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\notin span(S)
.
Assume to the contrary that 
char(F) \neq 2 \and
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix} 
\in span(S)
.
Then \exists c_1, c_2, c_3 \in F,
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix} 
=c_1
  \begin{pmatrix}
   1 & 0 \\
   -1 & 0 \\
  \end{pmatrix}
+c_2
  \begin{pmatrix}
   0 & 1 \\
   0 & 1 \\
  \end{pmatrix}
+c_3
  \begin{pmatrix}
   1 & 1 \\
   0 & 0 \\
  \end{pmatrix}
.
Then this system of linear equations has a solution:
(11)c_1+c_3=1
(21)-c_1=0
(12)c_2+c_3=0
(22)c_2=1.
When solving this system, we see that it has no solution.
This contradicts the assumption that it has a solution.
Then 
char(F) \neq2 \implies
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\notin span(S)
.
Then 
  \begin{pmatrix}
   1 & 0 \\
   0 & 1 \\
  \end{pmatrix}
\in span(S) \iff char(F)=2. Q.E.D.

A Field Problem

A Dimension Problem

Nikita