In the above equation, the subscripts p, q are used to indicate grid points in the volume representing the brain and the subscripts k, m are used to represent Cartesian coordinates x, y and z (i.e. they take values 1,2,3), d pq represents the Euclidean distances between the pth and qth grid points. The coefficients w j can be used to describe a column scaling by a diagonal matrix while σ i controls the spatial resolution. In particular, if σ i → 0 and w j = 1 the minimum norm solution described below is obtained.
