\begin{tikzpicture}[auto, node distance=2cm]
	\node[problem] (pz) {\problem{PZ}};
	\node[solution, right of=pz] (r-pz) {T/N};
	\node[left of=pz] (i-pz) {$A$};

	\node[problem, below of=pz] (mlzp) {\problem{MLZP$_d$}};
	\node[solution, right of=mlzp] (r-mlzp) {T/N};
	\node[left of=mlzp] (i-mlzp) {$k, L, I$};

	\draw[->, alpha, left] (i-pz) -- node[right] {$f$} (i-mlzp);

	\draw[->] (pz) -- (r-pz);
	\draw[->] (mlzp) -- (r-mlzp);

	\draw[->] (i-pz) -- (pz);
	\draw[->] (i-mlzp) -- (mlzp);

	\draw[Implies-Implies, double distance=2pt] (r-pz) -- (r-mlzp);
\end{tikzpicture}