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. 

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 < 
> <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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