The IMSL_CHNNDFAC function solves a real symmetric non-negative definite system of linear equations Ax = b. Computes the solution to Ax = b given the Cholesky factor.

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The factorization algorithm is based on the work of Healy (1968) and proceeds sequentially by columns. The i-th column is declared to be linearly dependent on the first i – 1 columns if:

where ε (specified in TOLERANCE) may be set. When a linear dependence is declared, all elements in the i-th row of R (column of L) are set to zero.

Modifications due to Farebrother and Berry (1974) and Barrett and Healy (1978) for checking for matrices that are not non-negative definite also are incorporated. The IMSL_CHNNDFAC procedure declares A to not be non-negative definite and issues an error message if either of the following conditions is satisfied:


Healy’s (1968) algorithm and the IMSL_CHNNDFAC procedure permit the matrices A and R to occupy the same storage. Barrett and Healy (1978) in their remark neglect this fact. The IMSL_CHNNDFAC procedure uses:

in condition 2 above to remedy this problem.

If an inverse of the matrix A is required and the matrix is not (numerically) positive definite, then the resulting inverse is a symmetric g2 inverse of A. For a matrix G to be a g2 inverse of a matrix A, G must satisfy conditions 1 and 2 for the Moore- Penrose inverse but generally fail conditions 3 and 4. The four conditions for G to be a Moore-Penrose inverse of A are as follows:

  1. AGA = A
  2. GAG = G
  3. AG is symmetric
  4. GA is symmetric


The symmetric nonnegative definite matrix in the initial example of IMSL_CHNNDSOL is used to compute the factorization only in the first call to IMSL_CHNNDFAC. Then, IMSL_CHNNDSOL is called with both the LLT factorization and the right-hand side vector as the input to compute a solution x.

RM, a, 4, 4
; Define the coefficient matrix.
row 0: 36 12 30 6
row 1: 12 20 2 10
row 2: 30 2 29 1
row 3: 6 10 1 14
PM, fac, Title = 'Factor', Format = '(4f12.3)'

IDL prints:

  6.000 2.000 5.000 1.000
  2.000 4.000 -2.000 2.000
  5.000 -2.000 0.000 0.000
  1.000 2.000 0.000 3.000
RM, b, 4, 1
; Define the right-hand side.
row 0: 18
row 1: 22
row 2: 7
row 3: 20
; Compute the solution and output.
x = IMSL_CHNNDSOL(b, Factor = fac)
PM, x, Title = 'Solution'

IDL prints:



IMSL_CHNNDFAC, A, Fac [, /DOUBLE] [, INVERSE=variable] [, TOLERANCE=value]



Two-dimensional matrix containing the coefficient matrix. Element A(i, j) contains the j-th coefficient of the i-th equation.


Matrix containing the LLT factorization of A.


DOUBLE (optional)

If present and nonzero, double precision is used.

INVERSE (optional)

Named variable into which the inverse of the matrix A is stored.

TOLERANCE (optional)

Tolerance used in determining linear dependence. Default: 100ε, where ε is machine precision.


Warning Errors

MATH_INCONSISTENT_EQUATIONS_2: Linear system of equations is inconsistent.

MATH_NOT_NONNEG_DEFINITE: Matrix A is not non-negative definite.

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See Also