Answer
$\lor^{9}_{i=1} \lor^{9}_{j=1} \lor^{9}_{n=1} P(i,j,n)$
Work Step by Step
We know that $P(i,j,n)$ asserts that the cell in row $i$ and column $j$ contains the number $n$. Now we can say that $\lor^{9}_{n=1} P(i,j,n)$ asserts that cell $(i, j)$ contains at least one number. Now to assert that each cell contains at least one number, we can take the conjunction of these statements over all cells :
$\lor^{9}_{i=1} \lor^{9}_{j=1} \lor^{9}_{n=1} P(i,j,n)$
