src/Math/GSeq.php
public function sum($n, $m = 0)
  {
    if($m > 0)
      return $this->sum($n+$m) - $this->sum($m);

    static::ensureValid($n);

    return $this->b * (1 - pow($this->q, $n)) / (1 - $this->q);
  }
public function sum($n, $m = 0)
  {
    if($m > 0)
      return $this->sum($n+$m) - $this->sum($m);

    static::ensureValid($n);

    return $this->b * (1 - pow($this->q, $n)) / (1 - $this->q);
  }
Calculate sum of N consequent characters.

Either N first, or N after first M.

@param int $n number of elements to sum up
@param ?int $m number of elements to skip
@return float the sum of elements