Columbia subject headings:
Differential operators--Congresses.
Singularities (Mathematics)--Congresses.
Harvard subject headings:
Differential equations, Hyperbolic.
Differential operators--Congresses.
Differential operators.
Singularities (Mathematics)
Singularities (Mathematics)--Congresses.
Princeton subject headings:
Differential operators--Congresses.
Singularities (Mathematics)--Congresses.