bandchol {mgcv} | R Documentation |

## Choleski decomposition of a band diagonal matrix

### Description

Computes Choleski decomposition of a (symmetric positive definite) band-diagonal matrix, `A`

.

### Usage

```
bandchol(B)
```

### Arguments

`B` |
An n by k matrix containing the diagonals of the matrix |

### Details

Calls `dpbtrf`

from `LAPACK`

. The point of this is that it has `O(k^2n)`

computational cost, rather than the `O(n^3)`

required by dense matrix methods.

### Value

Let `R`

be the factor such that `t(R)%*%R = A`

. `R`

is upper triangular and if the rows of `B`

contained the diagonals of `A`

on entry, then what is returned is an n by k matrix containing the diagonals of `R`

, packed as `B`

was packed on entry. If `B`

was square on entry, then `R`

is returned directly. See examples.

### Author(s)

Simon N. Wood simon.wood@r-project.org

### References

Anderson, E., Bai, Z., Bischof, C., Blackford, S., Dongarra, J., Du Croz, J., Greenbaum, A., Hammarling, S., McKenney, A. and Sorensen, D., 1999. LAPACK Users' guide (Vol. 9). Siam.

### Examples

```
require(mgcv)
## simulate a banded diagonal matrix
n <- 7;set.seed(8)
A <- matrix(0,n,n)
sdiag(A) <- runif(n);sdiag(A,1) <- runif(n-1)
sdiag(A,2) <- runif(n-2)
A <- crossprod(A)
## full matrix form...
bandchol(A)
chol(A) ## for comparison
## compact storage form...
B <- matrix(0,3,n)
B[1,] <- sdiag(A);B[2,1:(n-1)] <- sdiag(A,1)
B[3,1:(n-2)] <- sdiag(A,2)
bandchol(B)
```

*mgcv*version 1.9-1 Index]