[Rd] besselI inaccurate for negative integer order (PR#13556)

Jerry.Lewis at biogenidec.com Jerry.Lewis at biogenidec.com
Thu Feb 26 16:15:11 CET 2009

Full_Name: Jerry W. Lewis
Version: 2.8.1
OS: Windows XP Professional
Submission from: (NULL) (

It should be the case that
  besselI(x,-nu) == besselI(x,nu) == besselI(x,abs(nu))
for integer nu, yet R currently can return ridiculous values when nu is a
negative integer.

For instance, besselI(9.6,-44) returns -234626490 instead of the correct value
of 5.9041042646307223e-25, while besselI(9.6,44) gives essentially machine

This is more than an idle mathematical curiosity, since one consequence is that
dskellam in the VGAM package can return values <0 or >1.

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