We can expand $(k+i)^4$ using the binomial theorem:
$(k+i)^4 = k^4 + 4k^3i + 6k^2i^2 + 4ki^3 + i^4 = (k^4 - 6k^2 + 1) + (4k^3 - 4k)i$.
For the imaginary part to be 0, we must have $4k^3 - 4k = 0$. Factoring, we get $4k(k^2 - 1) = 0$, so $k = 0, 1, -1$. Thus, the
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