Method factorial

__test__/theorem.js:22–27

(n)

  	return new BigNumber(Math.exp(new BigNumber(n).toNumber()))
  }
  factorial(n) {
      if (new BigNumber(n).eq(0)) {
          return new BigNumber(1);
      }
      return new BigNumber(n).times(this.factorial(new BigNumber(n).minus(1)))
  }
  gamma (z) {
  	const g = 7;
  	const p = [