Figure 1: Example of comparison between our relation (7) and the ones (6), (12), (13).

The upper line is the sum of the variances (SV) . The black line is the lower bound (LB) given by our relation (7). The solid green line is the bound (6) (FB). The dashed green line is the bound (12) (PB1). The blue line is the bound (13) (PB2).