where I 0 is the intensity at R = 0 . The parameter n , called the "Sérsic index," controls the degree of curvature of the profile (see figure). The smaller the value of n , the less centrally concentrated the profile is and the shallower (steeper) the logarithmic slope at small (large) radii is. The equation for describing this is: d ln ⁡ ⁡ I d ln ⁡ ⁡ R = − − ( k / n ) R 1 / n .