From hwborchers at gmail.com  Fri Sep  9 20:55:43 2016
From: hwborchers at gmail.com (Hans W Borchers)
Date: Fri, 9 Sep 2016 20:55:43 +0200
Subject: [Rd] Different results for tan(pi/2) and tanpi(1/2)
In-Reply-To: <CAF8bMcbA8aToZg1KezJh7WYOYh-S9rByxbsd6m_AvMJugNkn-g@mail.gmail.com>
References: <CAML4n3NUKBMtXSt=naEktP4udKruenhG9N-Kdyx=sjpcx-BXvA@mail.gmail.com>
	<CAF8bMcbA8aToZg1KezJh7WYOYh-S9rByxbsd6m_AvMJugNkn-g@mail.gmail.com>
Message-ID: <CAML4n3P6duAC75ewwEtxaqsX6scygbstOjvn98AC7RUa0Mth=g@mail.gmail.com>

The same argument would hold for tan(pi/2).
I don't say the result 'NaN' is wrong,
but I thought,
tan(pi*x) and tanpi(x) should give the same result.

Hans Werner


On Fri, Sep 9, 2016 at 8:44 PM, William Dunlap <wdunlap at tibco.com> wrote:
> It should be the case that tan(pi*x) != tanpi(x) in many cases - that is why
> it was added.  The limits from below and below of the real function
> tan(pi*x) as x approaches 1/2 are different, +Inf and -Inf, so the limit is
> not well defined.   Hence the computer function tanpi(1/2) ought to return
> Not-a-Number.
>
> Bill Dunlap
> TIBCO Software
> wdunlap tibco.com
>
> On Fri, Sep 9, 2016 at 10:24 AM, Hans W Borchers <hwborchers at gmail.com>
> wrote:
>>
>> As the subject line says, we get different results for tan(pi/2) and
>> tanpi(1/2), though this should not be the case:
>>
>>     > tan(pi/2)
>>     [1] 1.633124e+16
>>
>>     > tanpi(1/2)
>>     [1] NaN
>>     Warning message:
>>     In tanpi(1/2) : NaNs produced
>>
>> By redefining tanpi with sinpi and cospi, we can get closer:
>>
>>     > tanpi <- function(x) sinpi(x) / cospi(x)
>>
>>     > tanpi(c(0, 1/2, 1, 3/2, 2))
>>     [1]    0  Inf    0 -Inf    0
>>
>> Hans Werner
>>
>> ______________________________________________
>> R-devel at r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>
>



[Rd] Different results for tan(pi/2) and tanpi(1/2)
Hans W Borchers