c is a sclar F(c)={|\nabla C|^q}/q

I want to know d(F(c))/dc

thx a lot

anyone can help me?

if you mean C = c in your functional, answer is zero

to clarify consider 1D case where \nabla c = c_x, therefore

F = F(c_x) (not F(c) !)

dF/dc = (dF/dc_x)(dc_x/dc),

dc_x/dc = d/dc(dc/dx)= d/dx(dc/dc)=d/dx(1)=0

hope this help.