In the context of control loops, specifically power supply control loops in my case, the symbol \$\phi_m\$ is used for phase margin. I've occasionally seen it with a capital M, but on balance it seems to be pretty consistently the lower case subscript.
My question is: Is there also a standard symbol for gain margin? Like perhaps \$A_m\$ or \$G_m\$? I searched around and could only find instances where it was actually spelled out; of course that could be because I could only search on "gain margin", not knowing exactly what to be looking for :)
It's come up as a value to be stated in my last couple of questions and I wanted to avoid using a phrase if a nice compact symbol would do the needful :)