  The numeraire portfolio in a financial market is the unique positive wealth
process that makes all other nonnegative wealth processes, when deflated by it,
supermartingales. The numeraire portfolio depends on market characteristics,
which include: (a) the information flow available to acting agents, given by a
filtration; (b) the statistical evolution of the asset prices and, more
generally, the states of nature, given by a probability measure; and (c)
possible restrictions that acting agents might be facing on available
investment strategies, modeled by a constraints set. In a financial market with
continuous-path asset prices, we establish the stable behavior of the numeraire
portfolio when each of the aforementioned market parameters is changed in an
infinitesimal way.
