Prove that the positive integers $n$ that cannot be written as a sum of $r$ consecutive positive integers, with $r > 1$, are of the form $n = 2^{l}$ for some $l \geq 0$.
Prove that the positive integers $n$ that cannot be written as a sum of $r$ consecutive positive integers, with $r > 1$, are of the form $n = 2^{l}$ for some $l \geq 0$.