float lower limit at given precision

[Jacopo Nespolo]

Hello everyone,
this is my first message here: I'm a student in theoretical physics at
University of Pisa, Italy.

I have just started my thesis, and I need to diagonalise a matrix with
great precision. Unfortunately, after some research, I didn't find any
suitable option ready to use, so I'm translating a couple of routines
of the GNU Scientific Library from double to mpf_t.

In one routine, I need to chop off the small elements from a vector of
mpf_t. The problem is I cannot understand what small means in this
case.  For those familiar with GSL, I need the equivalent of
GSL_DBL_EPSILON.
Hopefully some of you will know.

Thanks a lot,
Jacopo