We can then rearrange the matrix to solve directly for the heart voltages in terms of the measured body surface voltages,
Here the subscripts, T, V, and E stand for the nodes in the regions
of the torso, volume, and epicardium (surface of the heart), respectively.
In general, the 
sub-matrix is zero (except for very simple models
in which nodes connect directly from the torso to the heart). We can then
rewrite
(12)
as
which reduces to
where the matrix A contains all the geometry and conductivity values, the so-called transfer matrix.
(See exercise 8.)
