We face a rigidity problem for the fractional p-Laplace operator to extend to this new framework some tools useful for the linear case. It is known that (−∆) s (1 − |x| 2 ) s + and −∆p(1 − |x| p p−1 ) are constant functions in (−1, 1) for fixed p and s. We evaluated (−∆p) s (1 − |x| p p−1 ) s + proving that it is not constant in (−1, 1) for some p ∈ (1, +∞) and s ∈ (0, 1). This conclusion is obtained numerically thanks to the use of very accurate Gaussian numerical quadrature formulas.