The syntax for pde is
pde(u) [+-] op1(u)[*/]exp1 [+-] op2(u)[*/]exp2...=exp3
It defines a partial differential equation with non constant
coefficients where op
is one of the operator:
id()
dx()
dy()
laplace()
dxx()
dxy()
dyx()
dyy()
[*/]
means either a *
or a /
and similarly for laplace...dyy
.
Thus laplace(u)*(x+y) means