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## Delannoy Numbers

Delannoy numbers describe the number of paths from the southwest corner of a rectangular grid to the northeast corner, using only single steps north, northeast, or east. Delannoy numbers can be computed recursively using this formula:where

D(a,b) =D(a- 1,b) +D(a,b- 1) +D(a- 1,b- 1) [Weisstein 1999],D(0, 0) = 1.For a 1 x 1 grid, there are 3 paths:

For a 2 x 2 grid, there are 13 paths:

## Motzkin Numbers

The Motzkin numbers describe the number of paths from the southwest corner of a grid to the southeast corner, using only steps northeast, east, and southeast.2 x 2 grid, 2 paths:

3 x 3 grid, 4 paths:

4 x 4 grid, 9 paths:

5 x 5 grid, 21 paths:

Definitions and formulas cribbed from Eric Weisstein's CRC Concise Encyclopedia of Mathematics (CRC Press, 1999), pp. 411 and 1201. Figures designed and rendered using

Mathematica3.0.1.

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