“…These were new even for the case of the fractional Laplacian. As it could be expected, these general extension problems found many applications such as free boundary problems [2,4], fractional derivatives [11], master equations [14,38,92], fractional elliptic PDEs [29,95,100], fractional Laplacians on manifolds [7,28,31,39,46] and in infinite dimensions [74], symmetrization [40,45], nonlocal Monge-Ampère equations [70], numerical analysis [73], biology [94] and inverse problems [48,51].…”