public function isConvergent()
	{
		for ($i=$this->interval->getStart(); $i<$this->interval->getEnd(); $i++)
		{
			$diff = abs ($this->getValue ($i+1) - $this->getValue ($i));
			if (!$old_diff) $old_diff = $diff;
			else
			{
				if ($diff >= $old_diff)
					return false;
			}
		}
		return true;
	}