Columbia subject headings:
Differential equations, Elliptic.
Differential equations, Hyperbolic.
Differential equations, Nonlinear.
Harvard subject headings:
Differential equations, Elliptic.
Differential equations, Hyperbolic.
Differential equations, Nonlinear.
Princeton subject headings:
Differential equations, Elliptic.
Differential equations, Hyperbolic.
Differential equations, Nonlinear.