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

[Hans W Borchers]

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
>>
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>
>