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### LA_CHOLSOL

LA_CHOLSOL

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:

6u + 15v + 55w = 9.5

15u + 55v + 225w = 50

55u + 225v + 979w = 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.