The numeraire portfolio for unbounded semimartingales

Asset prices discounted by a tradable numeraire N should be (local) martingales under some measure Q that is equivalent to the original probability measure P. Instead of studying the set of equivalent martingale measures with respect to a prespecified numeraire, we will look for a tradable numeraire $N^P$ such that the discounted asset prices become martingales with respect to the original measure P. $N^P$ is called (P-)numeraire portfolio. Since the above martingale condition is too stringent to obtain a general existence result, we define a (generalized) numeraire portfolio by a weaker requirement. This $N^P$ is characterized as the solution to several optimization problems.