code: public static function create($b, $q, $n = 1)
    {
      if($n == 1)
        return new static($b, $q);
  
      static::ensureValid($n, "Amount of elements must be an integer number bigger than zero.");
  
      return new static($b * (1 - $q) / (1 - pow($q, $n)), $q);
    }
docs: Create new geometric sequence...
  
  [1] create($b, $q)
  - from base and multiplier;
  [2] create($sum, $q, $n)
  - from sum, multiplier and number of elements.