MathNet 2019-part-06 — 184 problems (EN–VI) Question 155

A set of integers $T$ is called *orensano* if there exist integers $a \lt b \lt c$ such that $a$ and $c$ belong to $T$ and $b$ does not belong to $T$. Find the number of subsets $T$ of $\{1,2, \ldots, 2019\}$ that are orensano.