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Bessel Function Evaluation Problem in Mathematica

  1. Jul 17, 2009 #1
    Hello,

    When I write:
    Code (Text):
    BesselK[1,2]
    in the Mathematica editor, the output is the same as the input. But I want to evaluate it numerically. In other words, I want the output be a number. How can I do that?

    Regards
     
  2. jcsd
  3. Jul 17, 2009 #2

    CompuChip

    User Avatar
    Science Advisor
    Homework Helper

    Did you try the N function?

    Code (Text):

    N[BesselK[1,2]]
    BesselK[1,2] // N
     
    In general, Mathematica only "evaluates" such functions for special arguments for which it knows exact values. In all other cases, it leaves the answer exact. You can force it to give a decimal representation using
    N[expr]
    or
    N[expr, # of decimals]


    Another way that often works is to give the arguments as floating numbers rather than exact values:
    Code (Text):

    BesselK[1., 2]
    BesselK[1, 2.]
    BesselK[1., 2.]
     
     
  4. Jul 17, 2009 #3
    Thank you CompuChip, but it didn't work, I still get the same expression. Why?

    Regards
     
  5. Jul 17, 2009 #4

    Hepth

    User Avatar
    Gold Member

    try again, the N method is correct.

    In[30]:= N[BesselK[1, 2]]

    Out[30]= 0.139866

    make sure its not misspelled.
     
  6. Jul 18, 2009 #5
    Now it is working, but believe me, I wrote it correctly, and the result was not produced numerically, I don't know why. Anyway, thank you Hepth very much to confirm the N[] numerical evaluation method.

    Regards
     
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