[SciPy-user] numpy 1.0b1: arccosh(0.0)

Pierre SCHNIZER p.schnizer at gsi.de
Fri Aug 4 07:29:31 CDT 2006


John Hassler wrote:
> It does exist - but it's a multivalued function, so you need a branch 
> cut.  See (halfway down the page):
> http://mathworld.wolfram.com/InverseHyperbolicCosine.html
> 
> john
> 
> David Cournapeau wrote:
> 

I agree. Perhaps that is the reason for the inf here?

<snip>
In [18]:numpy.arccosh(0+0j)
Out[18]:(-inf+0j)
</snip>
As it looks to me, thats not correct neither...

I am using it for transforming between elliptic coordinates and 
rectangular ones, there this behaviour is not that desireable.

Sincerely yours
	Pierre

>>Pierre SCHNIZER wrote:
>>  
>>
>>>Hello,
>>>
>>>	I don't know if that only happens on my machine....
>>>For numpy 1.0 b1 I get
>>>
>>><snip>
>>>In [241]:numpy.arccosh(0)
>>>Out[241]:nan
>>>    
>>>
>>I get the same results, but this is logical... acosh is defined by 
>>0.5(exp(z) + exp(-z)) for z complex, and this cannot be smaller than 1 
>>for z real, and never equal to 0, so arccosh(0) does not really make 
>>sense to me,
>>
>>cheers,
>>
>>David
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>>  
>>
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