MathDB

Let

\mathcal F

be the set of functions

f(x,y)

that are twice continuously differentiable for

x\geq 1

y\geq 1

and that satisfy the following two equations (where subscripts denote partial derivatives):

xf_x + yf_y = xy\ln(xy),

x^2f_{xx} + y^2f_{yy} = xy.

For each

f\in\mathcal F

, let

m(f) = \min_{s\geq 1}\left(f(s+1,s+1) - f(s+1,s)-f(s,s+1) + f(s,s)\right).

Determine

m(f)

, and show that it is independent of the choice of

f

B4