Method sigmoid

__test__/theorem.js:128–130

(x, n = 15)

  	return result.length == 1 ? result[0] : result
  }
  sigmoid(x, n = 15) {
  	return new BigNumber(new BigNumber(1).div(this.pow(this.c("e", n), x).plus(1)).toFixed(n))
  }
  sqrt(n) {
  	if (typeof n != 'object' || BigNumber.isBigNumber(n)) {
          n = [n]