Solution
= Solution
{c}
The family contains the whole space and the empty set. A finite intersection of nontrivial members is $(\max_j a_j,\infty)$. A nonempty union of rays is $(\inf_j a_j,\infty)$ when the infimum is finite, and is the whole line when the infimum is $-\infty$. Empty unions and unions involving the whole space are already included. Hence \b[$\tau$ is a <topology>]. This is the <upper-ray topology on the real line>.