Frechet differentiability via partial Frechet differentiability

Let $X_1,\dots ,X_n$ be Banach spaces and $f$ a real function on $X=X_1\times \cdots \times X_n$. Let $A_f$ be the set of all points $x\in X$ at which $f$ is partially Fr' echet differentiable but is not Fr' echet differentiable. Our results imply that if $X_1,\dots ,X_{n-1}$ are Asplund spaces and $f$ is continuous (resp. Lipschitz) on $X$, then $A_f$ is a first category set (resp. a $\sigma$-upper porous set). We also prove that if $X$, $Y$ are separable Banach spaces and $f:X\to Y$ is a Lipschitz mapping, then the set of all points $x\in X$ at which $f$ is G^ ateaux differentiable, is Fr' echet differentiable along a closed subspace of finite codimension but is not Fr' echet differentiable, is $\sigma$-upper porous. A number of related more general results are also proved.