A

= plant matrix

A _m

= coefficients in Fourier series of the describing function

a

= amplitude of signal into the describing function

a ^m

= simplex with label m

B

= control matrix

b

= magnitude of Bang-Bang relay or actuator

b _m

= intercept of mth segment with e = 0

C

= output state matrix, or three-dimensional manifold

C _n

= Abelian group

C*

= chain complex

?

= complex domain

c

= Fourier series angular coefficient

D

= direct feedthrough matrix, or two-dimensional manifold

d

= plant disturbance

DC

= direct current

d x

= time derivative of function x

E _D

= intermediate matrix in the H _? controller solution

e

= signal error(s)

e ^m

= exponential function of m

F

= intermediate matrix in the H _? controller solution

f( e)

= nonlinear distortion term

f _n

= Morse performance index function

G

= plant transfer function (augmented), or simplicial complex

g

= amplitude of error signal

H

= intermediate matrix in the H _? controller solution

H ₂

= 2-norm or average in Hardy space

H _?

= infinity-norm or supremeum in Hardy space

h...or ( h _m)

= coefficients of Fourier series in polar (rectangular) coordinates

I

= identity matrix or graph

?[K(a)]

= imaginary part of describing function

im( x)

= imaginary part of x

im _m

= mth quadrature component

J

= cost function

J _m