Blurb::
Singular radius
Description::
\c singular_radius works in conjunction with \ref method-nl2sol-singular_conv_tol
to test for underdetermined least-squares problems (stop when the relative
reduction yet possible in the sum of squares appears less than 
\c singular_conv_tol for steps of scaled length at most 
\ref method-nl2sol-singular_radius).

The internal default results in the internal use of steps of length 1. The 
default is selected with a value of -1.

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