BesselI[\[Nu], z] \[Proportional] Piecewise[{{1/Gamma[1 - \[Nu]]/(z/2)^\[Nu], Element[-\[Nu], Integers] && -\[Nu] > 0}}, (1/Gamma[1 + \[Nu]]) (z/2)^\[Nu]] /; (z -> 0)

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2007-05-02