## `combinat::stirling1` -- Stirling numbers of the first kind

#### Introduction

`combinat::stirling1`(n,k) computes the Stirling numbers of the first kind.

#### Call(s)

`combinat::stirling1(n,k)`

#### Parameters

`n,k` | - | nonnegative integers

#### Returns

an integer.

#### Details

- Let S(n,k) be the number of permutations of `n` symbols that have exactly `k` cycles. Then `combinat::stirling1`(n,k) computes -1^(n+k)*S(n,k).
- Let S1(n,k) be the stirling number of the first kind, then we have:

      sum(S1(n,k)*x^k, k=0...n) = x*(x-1) ... (x-n+1)

#### Example 1

Let us have a look what's the result of x*(x-1)*(x-2)*(x-3)*(x-4)*(x-5) written as a sum.

    >> expand(x*(x-1)*(x-2)*(x-3)*(x-4)*(x-5))

                        2                3       4       5    6
                   274 x  - 120 x - 225 x  + 85 x  - 15 x  + x

Now let us ``prove''  the formula mentioned in the ``Details'' section by calculating the proper stirling numbers

    >> combinat::stirling1(6,1);
       combinat::stirling1(6,2);
       combinat::stirling1(6,3);
       combinat::stirling1(6,4);
       combinat::stirling1(6,5);
       combinat::stirling1(6,6)

                                       -120

                                        274

                                       -225

                                        85

                                        -15

                                         1

#### Example 2

    >> combinat::stirling1(3,-1)

          Error:  Arguments must be nonnegative integers. [combinat::sti\
          rling1]

#### Further Documentation

J.J. Rotman, An Introduction to the Theory of Groups, 3rd Edition, Wm. C. Brown Publishers, Dubuque, 1988

#### Changes

- no changes

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