It is interesting FACT that given $l,m,n\geq 2$, there is (are) a *finite group* with elements $a,b$ such that $o(a)=l, o(b)=m$, and $o(ab)=n$ (see link for a nice example by Derek Holt / B. Sury).

Although, the group

$G_{l,m,n}=\langle a,b\colon a^l,b^m,(ab)^n\rangle$

contains elements with above property (?), it need not be finite. But for certain values of $l,m,n$, $G_{l,m,n}$ is finite (and these are interesting groups by their geometry):

$G_{2,2,n}\cong D_{2n}$,

$G_{2,3,3}\cong A_4$,

$G_{2,3,4}\cong S_4$,

$G_{2,3,5}\cong A_5$.

A natural question I would like to ask is:

For what values of $l,m,n$, the group $G_{l,m,n}$ is finite?

I wondered by the above FACT, and very useful discussion on it in **mathoverflow**, with nice answer by Derek Holt/ B. Sury. I tried to solve the question in the link, and for $(l,m,n)=(2,2,n)$, I quickly found that the group $D_{2n}$ is the best example for it.
Then I came up with the natural question above. This question may have been discussed with different point of view; but I didn't know too much about it.

Also, if possible, one may suggest(and edit) suitable title for this question.