<s>[INST] Proving a summation by induction How can I prove $$\sum_{i=2}^{n} \frac{1}{i^2-i}\lt1$$ by induction on $n$?
So far:
If $m$ is a natural number such that $m\ge2$, let $P(m)$ be the statement:
$$\sum_{i=2}^{m} \frac{1}{i^2-i}\lt1$$
We will prove $P(m)$ by induction on $m$.
Base Case:
P(2) is the statement:
$$\sum_{i=2}^{2} \frac{1}{2^2-2}=\frac{1}{2}\lt1$$
So $P(1)$ is true.
Inductive Step:
Let $k$ be a natural number such that $k\ge2$. Assume $P(k)$ for some arbitrary $k\ge2$.
$\sum_{i=2}^{n+1} \frac{1}{i^2-i}=\sum_{i=2}^{n} \frac{1}{i^2-i}+\frac{1}{(n+1)^2-(n+1)}\lt 1+\frac{1}{(n+1)^2-(n+1)}$ (by Ind. Hyp.).
I do not know where to go from here.
 [/INST] Well, since $\frac 1{(n+1)^2 -n} > 0$ and knowing $\sum\limits_{k=2}^n \frac 1{k^2 - k} = M < 1$, that isn't enough to prove $M + \frac 1{(n+1)^2 -n}< 1$.
We have to prove something a little stronger that $\sum\limits_{k=2}^n \frac 1{k^2 - k} $ is not just $< 1$ but less then $1-v_n$ for some $v_n > \frac 1{(n+1)^2 - n}$.
Let's figure what some of the differences are.
$\frac 1{2^2-2} = \frac 12 = 1-\frac 12$ so $v_1 = \frac 12$.
$\frac 1{2^2- 2} + \frac 1{3^2-3}=\frac 12 + \frac 16 = \frac 23$ and so $v_1 = \frac 13$.
$\sum\limits_{k=2}^4\frac 1{k^2-k}=\frac 23+\frac 1{12}=\frac 9{12}=\frac 34$ and $v_n = \frac 14$.
Can that possible be it?
Can it be true that $\sum\limits_{k=2}^n\frac 1{k^2-k}= 1-\frac 1n < 1$?
Well...... let's see:
We've done three base cases..
Induction step: $\sum\limits_{k=2}^n\frac 1{k^2-k}= 1-\frac 1n$ then
$\sum\limits_{k=2}^{n+1}\frac 1{k^2-k}=(\sum\limits_{k=2}^n\frac 1{k^2-k}) + \frac 1{(n+1)^2 - n}=$
$ 1-\frac 1n + \frac 1{(n+1)^2 - (n+1)} =$
$\frac {n-1}n + \frac 1{(n+1)((n+1)-1)} = \frac {n-1}n + \frac 1{n(n+1)}=$
$\frac {(n-1)(n+1)}{n(n+1)} + \frac 1{n(n+1)}=\frac {n^2-1}{n(n+1)} + \frac 1{n(n+1)}$
$\frac {n^2 -1+1}{n(n+1)} = \frac {n^2}{n(n+1)}=$
$\frac n{n+1} = 1 - \frac 1{n+1}$.
Excellent!  It works.
......
I should note;  A La Jose Carlos Santos excellent answer, that $\frac 1{n^2 - n} = \frac 1{n-1} - \frac 1n$.  That makes
$\sum_{k=2}^n \frac 1{n^2 - n}  = \sum_{k=2}^n(\frac 1{n-1} - \frac 1n)=(1 - \frac 12) + (\frac 12 -\frac 13) + (\frac 13 - \frac 14) + ......(\frac 1{n-1} - \frac 1n) = 1 - \frac 1n$.
Hence JC Santos's excellent answer.
It's a clever manipulation the first time you see $\frac 1{n^2 - n} = \frac {n- (n-1)}{n(n-1)} = \frac 1{n-1} - \frac 1n$ but one should get used to it.  It's more common than one would think.
It's essential in proving that $\lim_{n\to \infty}(1 + \frac 1n)^n:= e$ actually converges and exists.
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