1. A device for computing a function value of a function F, comprising: a memory storing a look-up table comprising a set of entries, each of which entries having associated with it a domain and an approximation function for approximating the function F on the associated domain; a truncator unit arranged to truncate or round a first value X1 to generate a second value X2; a selector unit, coupled to the truncator unit and the memory, and arranged to select an entry of the lookup-table according to the second value X2, thus selecting the approximation function that is associated with the selected entry; and an evaluator unit, coupled to the selector unit, and arranged to determine the function value of the selected approximation function at the first value X1.