= Invariant probability law of a Markov process
{title2=$\nu P_t=\nu\quad(t\ge0)$}
= Invariant probability measure of a Markov process
{synonym}
A <probability measure> is invariant for a <Markov process> with transition semigroup $P_t$ when starting with that law gives the same law at every later time. Equivalently, $\int P_tf\,d\nu=\int f\,d\nu$ for every bounded measurable function. Invariant laws may be constructed by <weak convergence of probability measures> of evolved laws when the <Feller semigroup> property permits passing the semigroup through the limit, as for the <extremal invariant measures of the contact process>.
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