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Real/Comlex function problem with D

• To: mathgroup at smc.vnet.net
• Subject: [mg74212] Real/Comlex function problem with D
• From: bar at ANTYSPAM.ap.krakow.pl
• Date: Wed, 14 Mar 2007 03:55:01 -0500 (EST)

Hi,
I have a complex function F[x]
When i defined:
ff[x_]:=Arg[F[x]]   (* i tried ComplexExpand[Arg[F[x]]] too *)

I obtainned good Plot[ff[x]] - its real - (no complex)

When i try 
ff1[x_]:=D[ff[x],x];
 I found ff1 is Complex funtion again !!

Why ?

(i tried Evaluate; Element[x,Reals]; FullSimplify - always the same)

Regards, Olaf