Re: Very strange behaviour of ArcTan[]
- To: mathgroup at smc.vnet.net
- Subject: [mg102673] Re: Very strange behaviour of ArcTan[]
- From: pfalloon <pfalloon at gmail.com>
- Date: Tue, 18 Aug 2009 06:08:59 -0400 (EDT)
- References: <h6bh4m$6ov$1@smc.vnet.net>
On Aug 17, 10:06 pm, Alexey <lehi... at gmail.com> wrote: > Hello, > I just have discovered some strange (buggy?) feature of ArcTan[] > function in Mathematica 5.2. Try the following; > > In[72]:= > ArcTan[.5,$MachinePrecision] > ArcTan[.5] > > Out[72]= > 1.53947 > > Out[73]= > 0.463648 > > The expression ArcTan[.5,$MachinePrecision] must (?) give an error but > in really gives some strange output without any error messages... What > do you think about this? You should check the documentation for the ArcTan function: there is a form of this function which takes two inputs, so all you have done is invoked that: In[29]:= {x,y} = {0.5, $MachinePrecision}; ArcTan[x,y] Out[30]= 1.53947 There is nothing buggy going on there, but clearly it's not what you meant to do. I'm not sure *what* you meant to do: if you were trying to get the output to MachinePrecision, the above is not the way to do this (not sure where you got the idea of adding a second argument: you need to use the function N). But it is unnecessary in this case because the argument 0.5 is already at machine precision. The following would be a sensible input: In[31]:= N[ArcTan[1/2]] Out[31]= 0.463648 Note that this returns machine-precision output: In[32]:= Precision[%] Out[32]= MachinePrecision Cheers, Peter.