We know that
"(-1) = \\cos (2n+1)\\pi \\pm i\\sin (2n+1)\\pi = e^{\\pm (2n+1)\\pi i}, \\text{for $n=1,2,3,\\cdots$}"
Therefore,
"(-1)^{2i} = e^{(\\pm(2n+1)\\pi i) \\cdot 2i}= e^{\\mp 2(2n+1)\\pi}, \\text{for $n=1,2,3,\\cdots$}\\\\\n(-1)^{2i} = e^{\\mp m\\pi}, \\text{for $m=2,4,6,\\cdots$}\\\\"
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