# Collecting proofs for $\sum_{n=2}^{\infty} \, \frac{n-1}{2^n} = 1$ [duplicate]

Update: The summation I came across has the form shown in title, and that exact question appears to be new. I could ask for proofs that take on this summation directly (without reducing it to summations starting at $$n = 0$$ or $$n =1$$), and that would be the preferred answer and the only one I will accept. But might as well let this fly as is, collecting all proofs of the identified equivalent variants.

I just stumbled across the fact that

$$\tag 1 \sum_{n=2}^{\infty} \, \frac{n-1}{2^n} = 1$$

This is equivalent to

$$\tag 2 \sum_{n=1}^{\infty} \, \frac{n}{2^n} = 2$$

I discovered $$\text{(1)}$$ using a 'matrix/combinatorial' argument, but it would need work to turn it into a formal proof.

I googled and found this Quora link, explaining how to show $$\text{(2)}$$.

I didn't find the question on this site, prompting this 'collecting proofs post':

Please supply a proof demonstrating either $$\text{(1)}$$ or $$\text{(2)}$$. If you use any theory or technique, mention that at the start of your answer.

One approach is to note that for $$S_k:=\sum_{n\geq k}\frac{1}{2^n}$$ you have $$S_0=2$$ and $$S_{k+1}=\frac{1}{2}S_k$$. Now rearranging terms in the summation yields $$\sum_{n\geq1}\frac{n}{2^n} =\sum_{n\geq1}\sum_{k=1}^n\frac{1}{2^n} =\sum_{k\geq1}\sum_{n\geq k}\frac{1}{2^k},$$ corresponding to the following picture: The inner sums equal the $$S_k$$, so we can simplify this to $$\sum_{k\geq1}S_k =\sum_{k\geq1}\frac{1}{2^k}S_0 =S_0\sum_{k\geq1}\frac{1}{2^k}=S_0S_1=2.$$

• Anyone care to explain the downvote? – Servaes Dec 12 '18 at 15:05
• Well not me! In fact, I can use your approach to get a direct solution to the $n \ge 2$ summation. I wind up with $S_2 \times S_0 = 1$. So don't feel bad - accepting and upvoting your answer! – CopyPasteIt Dec 12 '18 at 15:14
• Glad to hear that! And indeed the argument can be generalized (or repeated?) to evaluate $\sum_{n\geq 0}\frac{f(n)}{2^n}$ for any polynomial $f$. – Servaes Dec 12 '18 at 15:25
• This is a nice answer, but I think it is important to keep the answers to this question in one place. Users should search the site to avoid duplicating material already present. This applies with extra force to experienced users who should have a fairly good idea of what type of questions have already been covered. – Jyrki Lahtonen Dec 13 '18 at 6:03

A probability theory flavored approach: The expression $$\sum_{n\geq1} \frac{n}{2^n}$$ is also the expected number of IID fair coin flips it takes to gets a heads. Assuming you know this number is finite (by some root/ratio test business), let $$H$$ be the expected time. Then from conditioning on seeing heads or tails on the first flip, respectively, $$H$$ satisfies the recursion $$H = \frac{1}{2}(1) + \frac{1}{2}(H+1),$$ so $$H = 2$$.

A brute-force approach: by induction or otherwise, $$\sum_{k=1}^n \frac{k}{2^k} = 2 - \frac{n+2}{2^n}.$$ Sending $$n \to \infty$$ recovers the desired result.

The standard technique is $$\sum_{n\ge 1}r^n=\frac{1}{1-r}-1\implies\sum_{n\ge 1}nr^{n-1}=\frac{1}{(1-r)^2}$$.

We have that for $$|x|<1$$ by

$$f(x)=\sum_{k\ge1} x^n=\frac x{1-x} \implies f'(x)=\sum_{k\ge1} nx^{n-1}=\frac1{(1-x)^2}$$

therefore

$$\sum_{k\ge1} nx^{n}=x\cdot \sum_{k\ge1} nx^{n-1}=\frac x{(1-x)^2}$$

A solution by examining the differences between successive terms.

Let us define

$$S = \sum_{n=1}^{\infty} \, \frac{n}{2^n} = \sum_{n=1}^{\infty} u_n$$

Note (Edit): the definition $$u_n = \frac{n}{2^n}$$ will be used for $$n=0$$, i.e. for a term not involved in the series.

We have:

$$u_n - u_{n+1} = \frac{n-1}{2^{n+1}} = \frac{u_{n-1}}{4}$$

It follows immediately:

$$0 = S - S = \sum_{n=1}^{\infty} u_n - \sum_{n=1}^{\infty} u_{n+1} -u_1 = -u_1 + \frac{u_0}{4} + \frac{S}{4}$$

And therefore, noting that $$u_0=0$$ and $$u_1 = \frac{1}{2}$$ $$S = \sum_{n=1}^{\infty} \, \frac{n}{2^n} = 2$$

Note: the same procedure can be used to show that $$\sum_{k=1}^n \frac{k}{2^k} = 2 - \frac{n+2}{2^n}$$

• Your answer needs some work. One problem, $u_n - u_{n-1}$ is never a positive number. Also, $u_0$ is not defined. – CopyPasteIt Dec 12 '18 at 15:52
• Sorry, I just discovered a typo: it was $u_n - u_ {n+1}$. I will correct and detail my answer a little bit more – Damien Dec 12 '18 at 16:49
• and $u_n - u_{n-1} = \frac{n}{2^{n}} - \frac{n-1}{2^{n-1}} = \frac{2-n}{2^{n}} = \frac{-(n-2)}{2^{n-2}\times 2^2} = \frac{-u_{n-2}}{4}$ - three subscripts involved – CopyPasteIt Dec 12 '18 at 16:52
• I have corrected the typo. It should be more clear now – Damien Dec 12 '18 at 16:55
• For me, removing all mention of $u_0$ is the clearest exposition, but OK... – CopyPasteIt Dec 12 '18 at 17:02

I stumbled on this by realizing that since

$$\tag 1 \sum_{n=1}^{\infty} \, \frac{1}{2^n} = 1$$

it must be true that

$$\tag 2 (\sum_{n=1}^{\infty} \, \frac{1}{2^n}) \times (\sum_{n=1}^{\infty} \, \frac{1}{2^n}) = 1$$

Using the rearrangement approach (and the identity $$\sum_1^n \, 1 = n$$) found in Servaes' answer,

$$\tag 3 (\sum_{n=1}^{\infty} \, \frac{1}{2^n}) \times (\sum_{m=1}^{\infty} \, \frac{1}{2^m}) = \sum_{n+m =k}^{\infty} \, \frac{1}{2^k} = \sum_{k=2}^{\infty} \, \frac{k-1}{2^k} = 1$$

demonstrating the identity equation.