system/Pager/Pager.php
public function getPreviousPageURI(string $group = 'default', bool $returnObject = false)
	{
		$this->ensureGroup($group);

		$first = $this->getFirstPage($group);
		$curr  = $this->getCurrentPage($group);
		$page  = null;

		if (! empty($first) && ! empty($curr) && $first === $curr)
		{
			return null;
		}

		if ($first < $curr)
		{
			$page = $curr - 1;
		}

		return $this->getPageURI($page, $group, $returnObject);
	}
public function getPreviousPageURI(string $group = 'default', bool $returnObject = false)
	{
		$this->ensureGroup($group);

		$first = $this->getFirstPage($group);
		$curr  = $this->getCurrentPage($group);
		$page  = null;

		if (! empty($first) && ! empty($curr) && $first === $curr)
		{
			return null;
		}

		if ($first < $curr)
		{
			$page = $curr - 1;
		}

		return $this->getPageURI($page, $group, $returnObject);
	}
Returns the full URL to the previous page of results, or null.

@param string  $group
@param boolean $returnObject

@return string|null