On the equivalence of classical Helmholtz equation and fractional Helmholtz equation with arbitrary order

We show the equivalence of the classical Helmholtz equation and the fractional Helmholtz equation with arbitrary order. This improves a recent result of Guan, Murugan and Wei [Helmholtz solutions for the Fractional Laplacian and other related operators, to appear in Comm. Contemp. Math.].