## Modes

The goal of the library is to achieve double precision accuracy wherever possible. However the cost of evaluating some special functions to double precision can be significant, particularly where very high order terms are required. In these cases a mode argument allows the accuracy of the function to be reduced in order to improve performance. The following precision levels are available for the mode argument,

GSL_PREC_DOUBLE
Double-precision, a relative accuracy of approximately @c{$2 \times 10^{-16}$} @math{2 * 10^-16}.
GSL_PREC_SINGLE
Single-precision, a relative accuracy of approximately @c{$1 \times 10^{-7}$} @math{10^-7}.
GSL_PREC_APPROX
Approximate values, a relative accuracy of approximately @c{$5 \times 10^{-4}$} @math{5 * 10^-4}.

The approximate mode provides the fastest evaluation at the lowest accuracy.