def error(self, start, end):
        """Return the approximation cost on the segment [start:end].

        Args:
            start (int): start of the segment
            end (int): end of the segment

        Returns:
            float: segment cost

        Raises:
            NotEnoughPoints: when the segment is too short (less than ``'min_size'`` samples).
        """
        if end - start < self.min_size:
            raise NotEnoughPoints
        y, X = self.signal[start:end], self.covar[start:end]
        _, residual, _, _ = lstsq(X, y, rcond=None)
        return residual.sum()
Return the approximation cost on the segment [start:end].

        Args:
            start (int): start of the segment
            end (int): end of the segment

        Returns:
            float: segment cost

        Raises:
            NotEnoughPoints: when the segment is too short (less than ``'min_size'`` samples).
        
src/ruptures/costs/costautoregressive.py