AGN diagnostic developed by Smolcic et al. (2006, 2008) based on synthetic colors from a modified set of Stromgren uvby narrow (~200 Angstroms) bands. They adopt the filters: uz, vz, bz, yz, and show a sequence on the bz − yz versus vz − yz color-color plane. As shown in Figure 4 of Smolcic et al 2006, P1 is defined as the location along the sequence, defined by their equations 4-7, reproduced below.
On the blue side (vz − yz < 0.646):
P1 = 0.911 (c1 − 0.646) + 0.412 (c2 − 0.261)
P2 = −0.412 (c1 − 0.646) + 0.911 (c2 − 0.261)
On the red side (vz − yz > 0.646):
P1 = 0.952 (c1 − 0.646) + 0.307 (c2 − 0.261)
P2 = −0.307 (c1 − 0.646) + 0.952 (c2 − 0.261)
with the colors defined as c1 = vz − yz, and c2 = bz − yz.
The adopted dividing line is defined as: P1 > 0.15 (Smolcic et al. 2006).