Math 218: Matrices and Vector Spaces

This is the webpage for the Fall 2018 manifestation of Dr. Fitzpatrick's section of Math 218 at Duke University.

SageMath

SageMath is an open source computer algebra system written python.

Useful Commands

Use the syntax A = matrix([(1/2,-3,5),(-4/5,2,11),(6,-4,9)]) to define the matrix \[ A = \left[\begin{array}{rrr} \frac{1}{2} & -3 & 5 \\ -\frac{4}{5} & 2 & 11 \\ 6 & -4 & 9 \end{array}\right] \] Avoid using decimal notation! Input 1/2 instead of .5.

  • The command A.rref() computes the reduced row-echelon form of A.
  • The command A.det() computes the determinant of A, provided that A is square.
  • The command A.rank() computes the rank of A.
  • If A and B are \(m\times n\) matrices, then the command A+B computes the sum of A and B.
  • If c is a scalar, then the command c*A computes the scalar product of c and A.
  • If A is \(\ell\times m\) and B is \(m\times n\), then A*B computes the product of A and B.
  • The command A.transpose() computes the transpose of A.
  • The command A.is_symmetric() tests if A is symmetric.

Reduced Row-Echelon Form

The following code computes the reduced row-echelon form of a given matrix \(A\).

\(PA=LU\)-Factorization

The following code computes the \(LU\)-Factorization of a given matrix \(A\).

Reduced Row-Echelon form Practice!

Enter the number of rows and the number of columns of the matrix.

\(PA=LU\) Practice!

Enter the size of the matrix.

The Four Fundamental Subspaces

Enter the matrix.

To practice finding bases, you can generate a random \(m\times n\) matrix with the command A = random_matrix(ZZ, m, n).

A random \(m\times n\) matrix with rank \(r\) whose reduced row-echelon form has integer entries can be generated with the command A = random_matrix(ZZ, m, n, algorithm='echelonizable', rank=r).

Projections

The following code computes the projection matrix \(P\) onto the column space of the input matrix \(A\).

Least Squares Approximation

The following code computes the least squares approximation of a given list of points.

Gram-Schmidt

This code performs the Gram-Schmidt algorithm (without the normalization step) on the columns of the input matrix \(A\). The output is a matrix \(Q\) whose columns are produced by the Gram-Schmidt algorithm.

Pseudoinverses

Sage can compute the pseudoinverse of any matrix.