# pdClasses

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##### Positive-Definite Matrix Classes

Standard classes of positive-definite matrices (pdMat) structures available in the nlme library.

Keywords
models
##### Value

• Available standard classes:
• pdSymmgeneral positive-definite matrix, with no additional structure
• pdDiagdiagonal
• pdIdentmultiple of an identity
• pdCompSymmcompound symmetry structure (constant diagonal and constant off-diagonal elements)
• pdBlockedblock-diagonal matrix, with diagonal blocks of any "atomic" pdMat class
• pdNaturalgeneral positive-definite matrix in natural parametrization (i.e. parametrized in terms of standard deviations and correlations). The underlying coefficients are not unrestricted, so this class should NOT be used for optimization.

##### Note

Users may define their own pdMat classes by specifying a constructor function and, at a minimum, methods for the functions pdConstruct, pdMatrix and coef. For examples of these functions, see the methods for classes pdSymm and pdDiag.

pdCompSymm, pdDiag, pdIdent, pdNatural, pdSymm