Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Calculation/MathTrig.php
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();
    }
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();
    }
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