Thanks a lot, it's really clear to understand by the help of the website
you share!
Indeed, my problem is this:
1. I have fe which is a FiniteElement class, and I have fe_values which is
a FEValues class. I also have QGauss quadrature to create fe_values.
2. I iterate on every quadrature point in a iteration on every cell, store
the quadrature points.
3. Use fe_values.value(i, q) to calculate shape functions, and
fe.shape_value(i, point) to calculate another shape function, but find they
are different.
So I want to know what's the difference between this two methods? Because I
want to separate the fe and quadrature points, so can I use fe.shape_value
to calculate the shape functions?
Thanks again!
在 2018年5月6日星期日 UTC+8上午6:08:55，Denis Davydov写道：
>
> Hi,
>
> it's perfectly fine for a general FE shape functions to be negative at
> some points, that's clearly the case for quadratic
>
> http://hplgit.github.io/INF5620/doc/pub/sphinx-fem/._main_fem003.html#fem-approx-fe-fig-p2
>
>
> Denis.
>
> On Saturday, May 5, 2018 at 3:11:24 AM UTC+2, Zhao Yidong wrote:
>>
>> Hi everyone!
>> I use fe, which is FESystem class, but I find fe.shape_value( const
>> unsigned int i, const Point< dim > & p ) (the document is here:
>> https://www.dealii.org/8.5.0/doxygen/deal.II/classFE__Poly.html#a8c769a449d25d54e756fc67cca5dd30b)
>>
>> sometimes returns a negative number which seems not right.
>>
>> The Point I send into the function locates in the reference space ( [-1,
>> 1]x[-1, 1] ), I also try the Point in the real space, but also get negative
>> numbers sometimes.
>>
>> Thanks any response!
>>
>
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