// In mathematics, the binomial coefficient C(n, k) 

// is the number of ways of picking ‘k’ unordered outcomes

// from ‘n’ possibilities. It is given by the formula:  

// C (n, k) = n! / k! (n − k)!

​

// run the following test. . 

// bigint is not considered a basic type; it is an abbreviation for System.Numerics.BigInteger. 

​

// TEST 

// C 20 5

// Result: 15504       

​

let C (n:int) (k:int) =

    let N = bigint(n)

    let K = bigint(k)

    return fac N / fac K * fac N-K

let rec fac n =

    match n with

    | 0 | 1 -> 1

    | _ -> n * fac(n-1)

​

​