Columbia subject headings:
Differential equations, Partial--Congresses.
Differential equations--Congresses.
Perturbation (Mathematics)--Congresses.
Harvard subject headings:
Differential equations, Partial--Congresses.
Differential equations--Congresses.
Perturbation (Mathematics)--Congresses.
Princeton subject headings:
Differential equations, Partial--Congresses.
Differential equations--Congresses.
Perturbation (Mathematics)--Congresses.