[Maxima] Bug in tanh?
raymond.toy at ericsson.com
Mon Mar 19 10:02:42 CDT 2007
>>>>> "Barton" == Barton Willis <willisb at unk.edu> writes:
Barton> -----maxima-bounces at math.utexas.edu wrote: -----
>> I suspect it is sufficient to compute
>> Tanh = 2/(exp(-2*x)+1) -1
I think this doesn't work so well for small x.
Barton> In trigi.lisp cvs r 1.9, we had
Barton> I was largely responsible for the move toward turning over
Barton> most double float evaluation for trig-like functions to CL
Barton> (we all did debate the change on the list). If there is a
Barton> well-tested CL library for floating point evaluation of
Barton> trig-like functions, changing back to our own code
Barton> wouldn't be difficult.
I'm not aware of any such CL library. However, maxima already has a
bigfloat implementation of complex tanh. This could be modified
slightly to handle the real-valued case. I believe this version is
quite accurate for small x as well as for large x. This
implementation is based on Kahan's complex version.
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