, [also denoted by lim x!x0 lim y!y0 f ðx; yÞ and lim y!y0 lim x!x0 f ðx; yÞ respectively] are not necessarily equal. Although they must be equal if lim x!x0 y!y0 f ðx; yÞ is to exist, their equality does not guarantee the existence of this last limit.
, [also denoted by lim x! x0 lim y! y0 f ðx; yÞ and lim y! y0 lim x! x0 f ðx; yÞ respectively] are not necessarily equal. Although they must be equal if lim x! x0 y! y0 f ðx; yÞ is to exist, their equality does not guarantee the existence of this last limit.