mathjs/src/function/combinatorics/stirlingS2.js

import { factory } from '../../utils/factory'

const name = 'stirlingS2'
const dependencies = [
  'typed',
  'addScalar',
  'subtract',
  'multiplyScalar',
  'divideScalar',
  'pow',
  'factorial',
  'combinations',
  'isNegative',
  'isInteger',
  'larger'
]

export const createStirlingS2 = /* #__PURE__ */ factory(name, dependencies, (
  {
    typed,
    addScalar,
    subtract,
    multiplyScalar,
    divideScalar,
    pow,
    factorial,
    combinations,
    isNegative,
    isInteger,
    larger
  }
) => {
  /**
   * The Stirling numbers of the second kind, counts the number of ways to partition
   * a set of n labelled objects into k nonempty unlabelled subsets.
   * stirlingS2 only takes integer arguments.
   * The following condition must be enforced: k <= n.
   *
   *  If n = k or k = 1, then s(n,k) = 1
   *
   * Syntax:
   *
   *   math.stirlingS2(n, k)
   *
   * Examples:
   *
   *    math.stirlingS2(5, 3) //returns 25
   *
   * See also:
   *
   *    bellNumbers
   *
   * @param {Number | BigNumber} n    Total number of objects in the set
   * @param {Number | BigNumber} k    Number of objects in the subset
   * @return {Number | BigNumber}     S(n,k)
   */
  return typed(name, {
    'number | BigNumber, number | BigNumber': function (n, k) {
      if (!isInteger(n) || isNegative(n) || !isInteger(k) || isNegative(k)) {
        throw new TypeError('Non-negative integer value expected in function stirlingS2')
      } else if (larger(k, n)) {
        throw new TypeError('k must be less than or equal to n in function stirlingS2')
      }

      // 1/k! Sum(i=0 -> k) [(-1)^(k-i)*C(k,j)* i^n]
      const kFactorial = factorial(k)
      let result = 0
      for (let i = 0; i <= k; i++) {
        const negativeOne = pow(-1, subtract(k, i))
        const kChooseI = combinations(k, i)
        const iPower = pow(i, n)

        result = addScalar(result, multiplyScalar(multiplyScalar(kChooseI, iPower), negativeOne))
      }

      return divideScalar(result, kFactorial)
    }
  })
})