# Homework Help: Symmetric tridiagonal matrix

1. Feb 8, 2009

### gtfitzpatrick

The matrix A is symmetric and tridiagonal.
If B is the matrix formed from A by deleting the first two rows and columns, show that $$\left|A\right|$$ = a$$_{}11$$$$\left|M_{}11\right|$$ - (a$$_{}1$$)$$^{}2$$$$\left|B\right|$$

where $$\left|M_{}11\right|$$ is the minor of a$$_{}11$$

I know what a symmetric tridiagonal matrix is.
Is the minor oa a11 not just a11, the minor is the deterninant of a smaller part of a matrix right? but since a11 in only one entry is it not the minor as well?

i'm not sure where to start this...

2. Feb 8, 2009

### gtfitzpatrick

A = $$\begin{pmatrix}a11 & a12 & 0 & 0 & ... \\ a21 & a22 & a23 & 0 & ... \\ 0 & a32 & a33 & a34 & 0 & ...\\ 0 & 0 & a43 & a44 & a45 & ... \end{pmatrix}$$

B = $$\begin{pmatrix}a33 & a34 & 0 & 0 & ... \\ a43 & a44 & a45 & 0 & ... \\ 0 & a54 & a55 & a56 & 0 & ...\\ 0 & 0 & a65 & a66 & a67 & ... \end{pmatrix}$$

B = $$\begin{pmatrix}a22 & a23 & 0 & 0 & ... \\ a32 & a33 & a34 & 0 & ... \\ 0 & a43 & a44 & a45 & 0 & ...\\ 0 & 0 & a54 & a55 & a56 & ... \end{pmatrix}$$

a12$$^{}2$$ = a12 x a21 because its symetric

3. Feb 8, 2009

### Staff: Mentor

The minor of an entry in a matrix is the submatrix made up of all rows and columns that don't include that entry. For example, the minor $M_11$ of entry $a_11$ is the (n - 1) x (n - 1) matrix whose upper-left entry is $a_2$. A minor is a matrix, and is different from its determinant.

You're on the right track. Matrix B is as you show it in the first equation for B, with its upper-left entry of a33. I don't know what the other equation for B represents with its upper-left entry of a22.

To evaluate |A| by minors, you'll get a11 * M11 - a12 * M12, where M12 is the submatrix of all entries not in row 1 and column 2. The 1st column of M12 has only one nonzero entry in it: a21 (which by symmetry = a12). When you expand A12, going down the first column, you'll get a21 * |B|. Be sure to include the appropriate signs associated with a12 and a21.

Hope that helps

4. May 14, 2009