## Catalan numbers use case.

There are special numbers in combinatorial mathematics named after the French-Belgian mathematician Eugène Charles Catalan who studied the Towers of Hanoi puzzle.

The Catalan numbers are a sequence of natural numbers that usually describe recursively defined objects and calculated by formula:

Cn = ( ( 2n! ) / ( (n+1)! n! ) ) , where n is always non-negative.

An example for the first Catalan numbers for n = 0, 1, 2, 3, 4, 5 are 1, 1, 2, 5, 14, 42.

Today’s practical use cases of the Catalan numbers lay in the fields of computer science, geometry, GIS and geodesy, cryptography, and medicine.

## A recursive function to find n-th Catalan number with Python code:

```
def catalan(n):
if n <= 1:
return 1
res = 0
for i in range(n):
res += catalan(i) * catalan(n-i-1)
return res
# Check
N=6
for i in range(N):
print (catalan(i),end=",")
```

OUT: 1,1,2,5,14,42,