Explanation for intensive variables area interpolation

[iki77]

The code explain it like

For an intensive variable, the estimate at target polygon j is:

• v_j = \sum_i vi w{i,j}
• w{i,j} = a{i,j} / \sumk a{k,j}

And from what I understand is that:

• i = source
• j = target

What is k in \sumk a{k,j} and where does it come from?

[sjsrey]

k indexes all the source areas that contribute to target area j.