Solution
= Solution
When neither buys, X's <unilateral deviation> gains $2t/3-P$, while Y's gains $t/3-P$. For both to be nonpositive, $P$ must exceed the larger threshold. Hence
$$
\boxed{(nb,nb)\text{ is a Nash equilibrium iff }P\ge\frac{2n}{3(n+3)}.}
$$
Equality is included because a zero-gain deviation does not violate a <Nash equilibrium>. Consequently both joint-purchase and joint-nonpurchase <Nash equilibria> exist throughout $2t/3\le P\le t$, a coordination region.