The motivation of my question is the recent preprent of Armstrong, Rhoades and Williams <http://arxiv.org/abs/1305.7286> on rational Catalan combinatorics.


An important starting point of this paper is the fact that the number of lattice paths with steps $(1,0)$ and $(0,1)$ from the origin to $(m,n)$, where $gcd(m,n)=1$ weakly below the diagonal $y=\frac{n}{m} x$ equals
$$
\frac{1}{m+n}\binom{m+n}{m}.
$$


This can be proved (as done by Bizley, <http://www.jstor.org/discover/10.2307/41139633?uid=3737528&uid=2&uid=4&sid=21102085756863>) by considering all cyclic permutations of each path from the origin to $(m,n)$ and demonstrating that among these there is exactly one path below the diagonal.


I would like to know of any other proof of this fact, for example using generatingfunctionology. 


One approach I looked at (from the paper by Gessel and Ree <http://people.brandeis.edu/~gessel/homepage/papers/faber.pdf>) extends the recurrence $B(m,n)=B(m,n-1)+B(m-1,n)$ to all of $\mathbb N^2$ and then uses suitable initial values $B(m,0)$ and $B(0,n)$ to obtain generating functions for paths below $y=\frac{1}{m} x$. But it seems that the initial values for the general case are unpleasant.



The number of lattice paths below y=n/m x for gcd(m,n) = 1
You can use the approach of my paper *[A factorization for formal Laurent series](http://www.sciencedirect.com/science/article/pii/0097316580900746)*, 
Journal of Combinatorial Theory, Series A
28 (1980) 321-337. Although this problem is not considered in that paper, Theorem 4.1 and its proof give the following result: Let $m$ and $n$ be relative prime positive integers and let $p(k)$ be the number of paths from $(0,0)$ to $(km,kn)$ weakly below the diagonal $y=\frac{n}{m}x$. Expand 
$$\log\left(\frac{1}{1 -t^{n}x-t^{-m}y}\right)$$
as a power series in $x$ and $y$ and let $f$ be the constant term in $t$. Explicitly, we have
$$f= \sum\_{i=1}^\infty \frac{1}{(m+n)i} \binom{(m+n)i}{mi} x^{mi}y^{ni}.$$
Then 
$$\sum\_{k=0}^\infty p(k) x^{mk}y^{nk} = e^f.$$
This is equivalent to Grossman's formula, which was proved by Bizley. In particular, $p(1) = \binom{m+n}{m}/(m+n)$.
(Of course in the final result we could replace $x^my^n$ with a single variable.)
The method of this paper can also be used to count paths that stay strictly below (or above) the line $y=\frac{n}{m}x$.


