# bandcholsol¶

## Purpose¶

Solves the system of equations $$Ax = b$$ for x, given the lower triangle of the Cholesky decomposition of a positive definite banded matrix $$A$$.

## Format¶

x = bandcholsol(b, l)
Parameters: b (KxM matrix) – The right-hand side(s). l (KxN compact form matrix) – A Cholesky decomposition in compact (banded) form. x (KxM matrix) –

### Remarks¶

Given a positive definite banded matrix A, there exists a matrix L, the lower triangle of the Cholesky decomposition of A, such that $$A = LL'$$. l is the compact form of L; see band() for a description of the format of l.

b can have more than one column. If so, $$Ax = b$$ is solved for each column. That is,

$A*x[.,i] = b[.,i]$

## Examples¶

// Create matrix 'A' and right-hand side 'b'
A = { 1 2 0 0,
2 8 1 0,
0 1 5 2,
0 0 2 3 };
b = { 1.3, 2.1, 0.7, 1.8 };

// Create banded matrix form of 'A'
Aband = band(A, 1);

// Cholesky factorization of the banded 'A'
Lband = bandchol(Aband);

// Solve the system of equations
x = bandcholsol(b, Lband);


After the code above is run:

        0.000  1.000       1.495      1.300        1.300
Lband = 2.000  2.000  x = -0.098  b = 2.100  A*x = 2.100
0.500  2.179      -0.110      0.700        0.700
0.918  1.469       0.673      1.800        1.800