Assembling a single element of the stiffness matrix

variational formulation
1

I am trying to accelerate the computations of my solver. For this, I need to assemble only a few elements of the stiffness matrix. I can access the stiffness matrix itself using the commands:

a = #some bilinear form
A = assemble(a)
A_mat = A.array()

I only want to know the value A_mat located for example at [1,3] without computing the rest of the elements. Is this somehow possible?

Many thanks for the help

2

BTW, I know about assemble_local(a,c), where c is a cell index. However, I don’t know how the cell index translates to the element in the stiffness matrix.