# Greatest Power of Two not Divisor

## Lemma

Let $S = \set {1, 2, 3, 4, \ldots, n}$ be a set of integers from $1$ through $n$.

Let $2^t$ be the greatest power of $2$ in $S$.

Then $2^t$ does not divide any other integer in $S$.

That is, no other member of $S$ is a multiple of $2^t$.

## Proof

Let $k$ be a multiple of $2^t$ in $S$.

Then $k = 2^t \times \ell$ for some $\ell \in \Z, \ell \ge 1$.

If $\ell = 2$, then $k = 2 \times 2^t = 2^{t + 1}$, which would contradict $2^t$ being the highest power of $2$ in $S$.

Otherwise, if $\ell > 2$, then we would have $k = \ell \times 2^t > 2\times 2^t = 2^{t+1}$.

As $S$ contains all integers up to $k$, we would have $2^{t+1} \in S$, contradicting $2^t$ being the highest power of $2$ in $S$.

So $\ell = 1$, that is, $2^t$ is the only multiple of $2^t$ in $S$.

$\blacksquare$