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Strange error message from NIntegrate

• To: mathgroup at smc.vnet.net
• Subject: [mg28149] Strange error message from NIntegrate
• From: Henrik Holm <h.holm at spray.no>
• Date: Sun, 1 Apr 2001 00:08:08 -0500 (EST)
• Sender: owner-wri-mathgroup at wolfram.com

I am trying to do the following numerical integration:

  NIntegrate[x^{3/2}*Exp[-x]*BesselI[3/2,x/2], {x,0, Infinity}]

this results in the following error, 

  NIntegrate::"inum": 
   Integrand {0.141859424324175} is not numerical at {x} = {1.}.

I think this is very strange, after all, 0.1418... _is_ a
number.  It might be noted that doing the integration
symbolically works nicely:

  Integrate[x^{3/2}*Exp[-x]*BesselI[3/2,x/2], {x,0, Infinity}]

which gives the following result:

  16/(9*Sqrt[pi]) 


I guess I am doing something wrong, but I cannot understand
what.  Can anybody help me?



Regards, 
Henrik Holm