This should be a simple one but maybe I'm dumb or maybe I'm just tired, but how to prove that
$n = 1 + 2^1 + 2^2 + \cdots + 2^h$
is equal to
$n = 2^{h+1} - 1$
?
This should be a simple one but maybe I'm dumb or maybe I'm just tired, but how to prove that
$n = 1 + 2^1 + 2^2 + \cdots + 2^h$
is equal to
$n = 2^{h+1} - 1$
?
It’s a straightforward geometric series, as noted by rbm, but there are other ways to see it.
(1) Write it in binary: $2^n$ in binary is a $1$ followed by $n$ zeroes. Thus, in binary you’re adding $1+10+100+\ldots+1\underbrace{0\dots0}_h=\underbrace{1\dots1}_{h+1}\;.$ But clearly $\underbrace{1\dots1}_{h+1}+1=1\underbrace{0\dots0}_{h+1}$, which is the binary representation of $2^{h+1}$. Thus, $1+2^1+2^2+\ldots+2^h=2^{h+1}-1\;.$
(2) Prove it by induction on $h$. It’s certainly true for $h=0$: $1=2^1-1$. Suppose that for some $h\ge 0$ we have $1+2^1+2^2+\ldots+2^h=2^{h+1}-1\;.$ Then
$\begin{align*} 1+2^1+2^2+\ldots+2^h+2^{h+1}&=\left(1+2^1+2^2+\ldots+2^h\right)+2^{h+1}\\\\ &=\left(2^{h+1}-1\right)+2^{h+1}\\\\ &=2\cdot2^{h+1}-1\\\\ &=2^{(h+1)+1}-1\;, \end{align*}$
as desired.