thank for the reply!!
my fe degree is declared as "fe (FE_Q<dim>(degree+1), dim,FE_Q<dim>(degree),
1)" and I used "QGauss<dim> quadrature_formula(degree+2);" to calculate
integral over the cell.

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2016년 9월 20일 화요일 오전 9시 26분 6초 UTC-5, Praveen C 님의 말:
>
>
> On Tue, Sep 20, 2016 at 7:49 PM, JAEKWANG KIM <jaekw...@gmail.com
> <javascript:>> wrote:
>
>> Can I ask more about "the limits of our implementation of our quadrature
>> formulas?".
>> I wonder when it usually happens.
>>
>>
>> Once I calculated drag coefficient, with Q1, my error is 10% compare to
>> exact solution.
>> However, I can 1.7% error when I use Q2 which is significant decrease!
>> At Q3, I get 0.4% and At Q4 it starts to increase again 0.5%.
>> If I go higher, then my error is more than 100%..... I really want to
>> figure out why this happens....
>>
>> To summarize
>> From Q1~Q3... it shows significant decrease in error
>> but it is not anymore at Q4 and Q5
>>
>
> Hi
>
> If you are using FE_Q space with uniformly spaced support points, then
> there could be problem at higher degrees. Just to check, you should use
> Gauss nodes, e.g.
>
> FE_Q(QGaussLobatto<1>(degree+1))
>
> Best
> praveen
>
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