已知f(k)的z变换<img src="http://jgxm-question.oss-cn-beijing.aliyuncs.com/psst/files/e8ac264d0fca4d7f9946399b09e6c156.png?x-oss-process=image/sharpen,200"/>,若f(k)是因果序列，则F(z)的收敛域为