Bivariate optimal density

Question

Consider any feasible $p:[0,1]^2\to [0,1]$ that allows discontinuities and the problem

$$\min_{p(.)} \int_0^1\int_0^1 p(x,y)^2 dF(x) dG(y)$$

s.t. $$\int_0^1 p(x,y)dG(y)=k\phantom{0} for \phantom{0} a.e.\phantom{0} x\in [0,1]$$

where $F,G$ are distribution functions and $1/2<k<1$.

How could I solve the problem analytically?

Answer 1

By your condition on $p(x,y)$, we see that $p(x,y)=p(y)$ a.e.. Hence you just need to find $$ \min_p \int_0^1 p(y)^2 dG(y) $$ s.t. $$ \int_0^1 p(y) dG(y) =k $$ To minimize the functional, we perform a variation according to Lagrange multipliers. Take $p \to p + h \xi$ then we see first order terms in $h$ correspond to $$ \delta^{(1)} =\int_0^1 [2p(y) - \lambda ]\xi(y) dG(y)$$ To get a critical point, we need $p(y)=\lambda/2$ a.e. since we need $\delta^{(1)} =0 $ for all test functions $\xi$. But the constraint gives us that the solution is $$ p(x,y) = k \quad a.e. $$ For reference : Calculus of variations: Lagrange multipliers

Answer 2

Could I modify the above reasoning by first considering the relaxed problem,

$$ \min_p \int_0^1\int_0^1 p(x,y)^2 dG(y)dF(x) $$ s.t. $$ \int_0^1 \int_0^1 p(x,y) dG(y)dF(x) =k $$

Take $p \to p + h \xi$ then we see first order terms in $h$ correspond to $$ \delta^{(1)} =\int_0^1 \int_0^1 [2p(x,y) - \lambda ]\xi(x,y) dG(y)dF(x)$$ To get a critical point, we need $p(x,y)=\lambda/2$ a.e. Then,

$$ p(x,y) = k \quad a.e. $$

Since the solution is also feasible for the unrelaxed problem, we conclude the solution to the original problem is also the constant.