"Z^4-1=0\\\\\nZ^4=1\\\\\nZ^4=e^{2\\pi n}\\quad n=0,1,2,3\\\\\nZ=e^{\\frac{2\\pi n}{4}}\\quad n=0,1,2,3\\\\"
therefore,
"Z_1=1 \\\\\nZ_2=e^{\\frac{\\pi}{2}}=j\\\\\nZ_3=e^{\\pi}=-1\\\\\nZ_4=e^{\\frac{3\\pi}{2}}=-j\\\\"
"A=\\{1,j,-1,-j\\}=\\{z^0,z^1,z^2,z^3\\},\\quad z=j.\\\\\n\n\n\\therefore A=<z>=\\{z^k|k=0,1,2,3\\}"
Therefore set A is a cyclic group of order 4.
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