Unique continuation principles for a higher order fractional Laplace equationThe authors are partially supported by the INDAM-GNAMPA 2018 grant 'Formula di monotonia e applicazioni: problemi frazionari e stabilità spettrale rispetto a perturbazioni del dominio'. V Felli is partially supported by the PRIN 2015 grant 'Variational methods, with applications to problems in mathematical physics and geometry'. (3rd July 2020)