Obvious integer values are $a_1=1$, $a_2=a_3=2$. I'll show that these are the only ones.
First, we note that if $a_n
I'm sure there are numerous ways to prove that $a_n$ is strictly increasing for $n\ge3$, many of which will be purely technical. So far, my ideas have all centered around $a_n(a_n-1)\approx n-1/2$, which would in itself have sufficed to prove $a_n$ integral, and have gotten rather messy. I'll see if I can come up with a nice one.
Edit: I think I have a proof now that $a_n$ is strictly increasing for $n\ge3$.
Let $p_n(x)=x(x-1)-n$. This is increasing for $x\ge1/2$ and has positive root $x_n=1/2+\sqrt{n+1/4}$. We can then do induction on $x_{n-1}
Since $x_n(x_n-1)=n$ and $a_n(a_{n+1}-1)=n$, if $a_nx_n$.
if $a_n>x_{n-1}$, we get
$$a_{n+1}-1=\frac{n}{a_n}<\frac{n}{x_{n-1}}n$ which can be shown for $n>1$ by plugging in the values.
Since $x_n$ are increasing and $x_3
How I got the idea in the first place?
I computed $a_n$ numerically (using Maple), and quickly found that $a_n(a_n-1)\approx n-1/2$. This lead me to think of proving that $a_n(a_n-1)$ was not an integer since this seemed to be true by a large margin. The brute force approach, which is what I started out trying, would have been to show this by proving the approximation was sufficiently accurate. However, since I had already observed that $a_n$ was increasing, comparing $x_n(x_n-1)$ to $x_n(x_{n+1}-1)=n$ the way I did was quite apparent.