       RE: Rank of a matrix depending on a variable

• To: mathgroup at smc.vnet.net
• Subject: [mg122013] RE: [mg122000] Rank of a matrix depending on a variable
• From: "David Park" <djmpark at comcast.net>
• Date: Sun, 9 Oct 2011 03:51:29 -0400 (EDT)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com
• References: <26711112.27416.1318065689598.JavaMail.root@m06>

```The problem here is that RowReduce is at once more general and less general
than needed. It is more general in incorporating sophisticated numerical
techniques, but less general in the ability to confine its operation to a
submatrix of a larger matrix but carry the row operations over to all
columns.

Nevertheless, there is a simple way to do this in regular Mathematica.

(A = {{1, 0}, {2, 0}}) // MatrixForm
(B = {{a}, {2}}) // MatrixForm

Calculate the null space of the transpose of A and then apply this to B. To
be consistent all of the resupting elements must be set to zero so this
gives a set of equations in the parameters.

NullSpace[Transpose@A]
%.B
{{-2, 1}}
{{2 - 2 a}}

giving the equation 2 - 2 a == 0.

A second method is to use GroebnerBasis. Let the variable symbols be x1 and
x2. We then write the equations as polynomials that are equal to zero and
find a Groebner basis on x1 (x2 doesn't enter).

GroebnerBasis[{x1 - a, 2 x1 - 2}, {x1}]
{-1 + a, -1 + x1}

giving a == 1 and x == 1 for the solution.

This is fine and good if one understands NullSpace and the fact that one has
to use the Transpose of the A matrix, or if one is into Groebner bases.

Another approach can be used with the Students Linear Equations section of
the Presentations application. This allows us to set up a matrix structure
representing linear equation, with named rows and columns and dividers. The
matrix structure can be displayed in a separate window (or in the notebook)
and manipulated step by step. The following does your example. There are
routines to compose a matrix structure, operate on it and extract various
parts.

Unfortunately I can't easily copy the matrix structure display form into the
email. A PDF and Mathematica notebook showing the complete solutions will be
posted at the archive site Pete Lindsay at the St. Andrews Mathematics
department keeps for me, perhaps with a day or so delay.

http://home.comcast.net/~djmpark/DrawGraphicsPage.html

Here is the code for one method using a null space.

1) Define case1 as a matrix structure with 2 rows and 3 columns.
2) Insert column names, which can actually be variable symbols or constants.
3) Add a divider after the second row.
4) Insert matrix A starting at row 1, column1.
5) Insert matrix B starting at row 1, column 3.
6) Display the structure.

<< Presentations`

case1 = LECreate[2, 3];
LEInsertColumnNames[case1, 1, {x1, x2, 1}]
LEColumnDividers[case1, {2}]
LEInsertMatrix[case1, {1, 1}, A]
LEInsertMatrix[case1, {1, 3}, B]
LEPrint[case1]

(This displays a matrix with row and column names and dividers.)

The following prints the equations: Notice that the expressions are obtained
by dotting the row of column names with the row of elements.

LERowExpression[case1, #, 1 ;; 2] ==
LERowExpression[case1, #, 3 ;; 3] & /@ Range //
Column[#, Alignment -> "=="] &

X1 == a
2 x2 == 2

The following reduces the lhs matrix and carries the row operations over to
the rhs without attempting to carry the reduction into the rhs. We then
scale the last row to remove a common factor of 2. (The matrix itself is the
first part of case1.)

LEMakeEchelonForm[case1, {1, 1}, {2, 2}]
LERowDividers[case1, {1}]
LEScaleRow[case1, 1/2, 2]; case1 = MapAt[Expand, case1, 1];
LEPrint[case1]

(Again a display of the reduced matrix structure.)

The all zero rows on the lhs generate the null space. For a solution to
exist the corresponding rhs entries must be zero, which supplies the
condition on the parameter(s). Writing the equations again, we obtain:

LERowExpression[case1, #, 1 ;; 2] ==
LERowExpression[case1, #, 3 ;; 3] & /@ Range
Solve[%][]

{x1 == a, 0 == 1 - a}
{a -> 1, x1 -> 1}

Student's Linear Equations has routines for composing matrix structures,
routines for row operations on the structure, routines for extracting
sub-matrices, row or column expressions in equation form, and routines for
displaying the matrix structure in the notebook, or in a separate window
where it is dynamically updated. Row, column or element positions can be
pasted into the notebook from the displays. By using row and column names
the matrices are displayed in a context. There are also routines for
displaying rows as reaction equations for use in chemical stoichiometry and
operations for basic linear programming. It also has provision for covariant
(the usual) and contravariant columns. The capability is useful for didactic
purposes and for solving small to medium size problems where one wishes to
see the steps.

David Park
djmpark at comcast.net
http://home.comcast.net/~djmpark/

From: Mikel [mailto:ketakopter at gmail.com]

Hi,

A bit of background for my problem: I'm trying to solve a linear system of
equations in matrix form, A.x==B. I have the A matrix, which is square
and singular. I also have the B vector, which depends on some parameters (or
variables, I'm not sure how to call it). I'm looking for the value of the
parameters, so there exists solution to the system. In other words, I want
the rank of the augmented matrix AB to be equal to the rank of A.

I thought about using the MatrixRank function, but it does not support
parameters, it seems. Then I tried RowReduce, but even though in the first
example of the help page the result is given with parameters, I only get
numbers.

Here's an example to try:

============

Clear[a]
A = {{1, 0}, {2, 0}};
B = {{a}, {2}};
AB = ArrayFlatten[{{A, B}}];
MatrixRank[A]
MatrixRank[AB]
RowReduce[AB]

a = 1;
MatrixRank[A]
MatrixRank[AB]
RowReduce[AB]

============

As can be seen, the rank does depend on the value of the "a" parameter, but
RowReduce just outputs a numeric value.

So, the questions are:
1) Why does RowReduce not give a parameter in the result? Is this a bug?
2) Is there any other way to solve the problem, i.e. to get the rank of AB
as a function of the "a" variable?

I'm using Mathematica 7, Student version, for what is worth.

```

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