The LA_CHOLSOL function is used in conjunction with the LA_CHOLDC to solve a set of *n* linear equations in *n* unknowns, *AX* = *B*, where *A *must be a symmetric (or Hermitian) positive-definite array. The parameter *A* is input not as the original array, but as its Cholesky decomposition, created by the routine LA_CHOLDC.

The LA_CHOLSOL function may also be used to solve for multiple systems of linear equations, with each column of *B* representing a different set of equations. In this case, the result is a *k*-by-*n* array where each of the *k* columns represents the solution vector for that set of equations.

LA_CHOLSOL is based on the following LAPACK routines:

Output Type |
LAPACK Routine |

Float |
spotrs |

Double |
dpotrs |

Complex |
cpotrs |

Double complex |
zpotrs |

## Examples

Given the following system of equations:

6*u* + 15*v* + 55*w* = 9.5

15*u* + 55*v* + 225*w* = 50

55*u *+ 225*v *+ 979*w* = 237

The solution can be derived by using the following program:

`; Define the coefficient array:`

a = [[6.0, 15.0, 55.0], $

[15.0, 55.0, 225.0], $

[55.0, 225.0, 979.0]]

`; Define the right-hand side vector b:`

b = [9.5, 50.0, 237.0]

`; Compute the Cholesky decomposition of a:`

`achol = a ; make a copy`

`LA_CHOLDC, achol`

`; Compute and print the solution: `

`x = LA_CHOLSOL(achol, b)`

PRINT, 'LA_CHOLSOL solution:', x

When this program is compiled and run, IDL prints:

LA_CHOLSOL Solution:

-0.499999 -1.00000 0.500000

The exact solution vector is [-0.5, -1.0, 0.5].

## Syntax

*Result *= LA_CHOLSOL( *A*, *B *[, /DOUBLE] [, /UPPER] )

## Return Value

The result is an *n*-element vector or *k*-by-*n* array.

## Arguments

### A

The* n*-by-*n* Cholesky factorization of an array, created by the LA_CHOLDC procedure.

### B

An *n*-element input vector containing the right-hand side of the linear system, or a *k*-by-*n* array, where each of the *k* columns represents a different linear system.

## Keywords

### DOUBLE

Set this keyword to use double-precision for computations and to return a double-precision (real or complex) result. Set DOUBLE = 0 to use single-precision for computations and to return a single-precision (real or complex) result. The default is /DOUBLE if *A* is double precision, otherwise the default is DOUBLE = 0.

### UPPER

Set this keyword if *A* contains the upper triangular array, rather than the lower triangular array.

*Note: *If the UPPER keyword is set in the LA_CHOLDC then the UPPER keyword must also be set in LA_CHOLSOL.

## Version History

5.6 |
Introduced |

## Resources and References

For more details, see Anderson et al., *LAPACK Users' Guide*, 3rd ed., SIAM, 1999.