function digamma( x ) {
	var g;
	var r;
	g = x - root1;
	g -= root2;
	g -= root3;
	r = rateval( x-1.0 );
	return (g*Y) + (g*r);
}
Evaluates the digamma function over interval `[1,2]`.

@private
@param {number} x - input value
@returns {number} function value