A square-root factor of the positive-definite matrix represented by
object is obtained. Letting S denote a positive-definite matrix, a square-root factor of S is any square matrix L such that . This function extracts L.
a vector with a square-root factor of the positive-definite matrix associated with
object stacked column-wise.
Pinheiro, J.C., and Bates, D.M. (2000) "Mixed-Effects Models in S and S-PLUS", Springer.
This function is used intensively in optimization algorithms and its value is returned as a vector for efficiency reasons. The
pdMatrix function can be used to obtain square-root factors in matrix form.
Documentation reproduced from package nlme, version 3.1-127. License: GPL (>= 2)