code: public static function SERIESSUM(...$args)
      {
          $returnValue = 0;
  
          // Loop through arguments
          $aArgs = Functions::flattenArray($args);
  
          $x = array_shift($aArgs);
          $n = array_shift($aArgs);
          $m = array_shift($aArgs);
  
          if ((is_numeric($x)) && (is_numeric($n)) && (is_numeric($m))) {
              // Calculate
              $i = 0;
              foreach ($aArgs as $arg) {
                  // Is it a numeric value?
                  if ((is_numeric($arg)) && (!is_string($arg))) {
                      $returnValue += $arg * pow($x, $n + ($m * $i++));
                  } else {
                      return Functions::VALUE();
                  }
              }
  
              return $returnValue;
          }
  
          return Functions::VALUE();
      }
docs: SERIESSUM.
  
  Returns the sum of a power series
  
  @param float $x Input value to the power series
  @param float $n Initial power to which you want to raise $x
  @param float $m Step by which to increase $n for each term in the series
  @param array of mixed Data Series
  
  @return float