Many years ago, I discovered the remarkable array (apparently originally discovered by Ramanujan)

```
1
1 3
2 10 15
6 40 105 105
24 196 700 1260 945
```

which is defined by $S(i,j) = i\ S(i-1,j) + (i+j)\ S(i-1,j-1)$ and $S(0,1)=1$, and $S(i,j)=0$ if $j<1$ or $j>i+1$. This array has the remarkable property that the sum of the numbers in the $i$'th row is $(i+1)^{i+1}$. This is not easy to prove. There are three approaches I know to proving this

- Generating functions.
- Counting subclasses of labeled trees.
- Generalizing to a 3-dimensional array of numbers. There are recurrences on two sets of parallel planes, which intersect in the rows. One set of parallel planes contains the array above, and the other set contains a recurrence from which one can immediately deduce the row sums. Proving that these two different sets of recurrences give the same array is straightforward (albeit tedious without computer algebra) using induction.

(See *SIAM Review*, Problems and Solutions column, Vol. 21, pp. 258-260 (1979).)

The third approach is reminiscent of Wilf and Zeilberger's A = B theory of combinatorial identities, except there you have 3-dimensional arrays with recurrences on *three* sets of parallel planes. Wilf and Zeilberger's theory does not appear to shed any light on this recurrence.

My question is: does anybody know any other 3-dimensional arrays which have recurrences on two sets of parallel planes, but which do not fall under the A = B theory (so you cannot find a recurrence on a third set of parallel planes)? I would especially be interested in recurrences whose coefficients are polynomials in the coordinates $i,j,k$.

For more information about the connection with labeled trees, although this isn't directly connected with my question, see the papers Chen and Guo, Bijections behind the Ramanujan polynomials and Guo and Zeng, A generalization of the Ramanujan polynomials and plane trees, as well as the references in them.