Bessel Function -- from Wolfram MathWorld

A function Z_n(x) defined by the recurrence relations Z_(n+1)+Z_(n-1)=(2n)/xZ_n (1) and Z_(n+1)-Z_(n-1)=-2(dZ_n)/(dx). (2) The Bessel functions are more frequently defined as solutions to the differential equation x^2(d^2y)/(dx^2)+x(dy)/(dx)+(x^2-n^2)y=0.

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