Hello,
In fact, it seems that in Sage 7.2, cos((p/q)*pi) gets reduced mod pi
instead of 2pi, at least in cases where it does not simplify to square
roots:
sage: [cos(pi*(i+13)/13) - cos(pi*i/13)
for i in range(0,50) if i%13!=0] == [0]*46
True
It would be good to know what fixed that bug. It might be a Ginac bug.
Perhaps cos(14*pi/9) should be added as a test of the cos function?
Bruno
On 09/16/2017 07:43 AM, John Cremona wrote:
> You should upgrade to a more recent version:
>
> ┌────────────────────────────────────────────────────────────────────┐
> │ SageMath version 8.0, Release Date: 2017-07-21 │
> │ Type "notebook()" for the browser-based notebook interface. │
> │ Type "help()" for help. │
> └────────────────────────────────────────────────────────────────────┘
> sage: float(cos(14*pi/9))
> 0.17364817766693041
>
>
>
> On 16 September 2017 at 12:27, Mickael Pechaud <mickaelpech...@gmail.com>
> wrote:
>> Hello,
>>
>> I'm running SageMath 7.2, on a 64bits laptop with a Ubuntu 16.04.2 LTS
>> (xenial). I got a wrong answer from cos :
>>
>> sage: float(cos(14*pi/9))
>> -0.17364817766693041
>>
>> It should be positive - and it's inconsistent with the following lines :
>>
>> sage: float(cos(14*pi/9-2*pi))
>> 0.17364817766693041
>>
>>
>> sage: float(14*pi/9)
>> 4.886921905584122
>>
>> sage: float(cos(4.886921905584122))
>> 0.17364817766692953
>>
>> Am I missing something, or is there an actual problem here ?
>>
>> Thanks in advance,
>>
>> Mickaël
>>
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