sPHENIX tracking discussion

[David Morrison <dave AT bnl.gov>]

Hi Tom,

FWIW, I was able to reproduce (within my eye’s ability to tell) Fig 5.2 from 
Stefan’s thesis.  Seems promising.

Cheers,
Dave

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> On Dec 7, 2015, at 11:17 AM, Thomas K Hemmick 
> <hemmick AT skipper.physics.sunysb.edu> wrote:
> 
> Hi Dave
> 
> Thanks very much!  I found 831 in fortran Friday afternoon and used extern 
> statements to make it callable from C++.  Even though fortran supports 
> things like functions, 831 is written with the return value as one of the 
> arguments.  I'll be delighted to switch to the fable output as it might 
> make the code much more elegant.
> 
> Thanks!
> Tom
> 
> On Sun, Dec 6, 2015 at 3:32 PM, David Morrison <dave AT bnl.gov> wrote:
> Hi all,
> 
> Tom mentioned that he was looking for a C++ implementation of the modified 
> Bessel functions of (purely) imaginary order to carry out the E_phi 
> calculations from Stefan Rossegger’s thesis for different boundary 
> conditions.
> 
> Who would’ve guessed there were any new developments in the calculation of 
> special functions any more?
> 
> It looks like ACM TOMS algorithm 831(2004) is what you want.  Yes, it’s in 
> Fortran 77, but that may be OK.  The sources can be downloaded from here 
> http://www.netlib.org/toms/831.  The source compiles fine and there’s an 
> included driver to check the output against values provided by the authors. 
>  I see a few differences in the output – most (but not all) of them seem to 
> be deep in the decimals.  I converted the F77 to C++ using fable 
> (http://cci.lbl.gov/fable/) which worked mostly fine – I needed to address 
> a couple of complaints from the compiler.  That then compiles fine as C++.  
> I used the C++ to plot Ki(x) and compare to what I get from Wolfram Alpha – 
> those are attached.  Looks OK.  Maybe this is a bit of a run-around for 
> something that “is of second order importance compared to E_r”, but it can 
> be made to work.
> 
> Cheers,
> Dave

David Morrison  Brookhaven National Laboratory  phone: 631-344-5840
                                Physics Department, Bldg 510 C  email: 
dave AT bnl.gov
                                Upton, NY 11973-5000

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