Bonjour, Thank you for you reply.
I calculate the jabobian matrix using J=[(x_node(2)-x_node(1))/2 0; 0 (y_node(4)-y_node(1))/2]. This is a special case for rectangle element (not applicable for general quadrilateral element). If I calculate like this, all the terms will be zeros when the ordering changes. I have talked with Jeremy and I will change this formula to adapt to more general cases. Li Nan
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