strange besselj behavior

[Ahn Kyung]

Folks,

Test this.

x=linspace(0,100,10000)';
plot(x, besselj(0,x));
plot(x, besselj(2,x));

When I tried this, besselj(0,x) did not have any problem, but 
plot(x,besselj(2,x)) spit out

octave:4> plot(x,besselj(2,x))

gnuplot> pl '/tmp/oct-PJiXlG' t "line 1"
           ^
        line 0: Bad data on line 5

So I checked what besselj(2,x) looked like. Here's the comparison with 
besselj(0,x):

octave:5> besselj(0,x)
ans =

   1.0000e+00
   9.9997e-01
   9.9990e-01
   9.9977e-01
   9.9960e-01
   9.9937e-01
   9.9910e-01
        ...

octave:6> besselj(2,x)
ans =

  0.00000 - 0.00000i
  0.00001 + 0.00000i
  0.00005 + 0.00000i
  0.00011 + 0.00000i
  0.00020 + 0.00000i
  0.00031 + 0.00000i
  0.00045 + 0.00000i

---------------

So, besselj in some cases(besselj(3,x)->bad; besselj(4,x)->OK, etc.) thinks 
that it can have imaginary values like Hankel functions?? The plot looked 
OK when I tried plot(x,real(besselj(2,x))), so at least I may trust the 
result with real(..).

I tried this just to see how well besselj would work. I'm using 
octave-2.1.50, installed via rpm on redhat core 2 platform. Well, I also 
have a Mandrake 10 box with octave installed with mandrake specific rpm and 
guess what. The same problem.

If any of you experience no such problems in other versions or other 
platforms, it may be just some bad compilation. Otherwise, I'd go ahead and 
report it as a bug.

So I'd appreciate if you could share your experience on this.




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