Blurb::
Singular convergence tolerance
Description::
\c singular_conv_tol
(NL2SOL's \c sctol) is the singular convergence tolerance, which works
in conjunction with \ref method-nl2sol-singular_radius to test for
underdetermined least-squares problems (stop when the relative
reduction yet possible in the sum of squares appears less then \c
singular_conv_tol for steps of scaled length at most \ref method-nl2sol-singular_radius).

The internal default is a function of machine epsilon 
(as limited by \c function_precision). The default is selected with a value of -1.

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