# How to select the sign of the square root of each element of a DFT in obtaining the square root of a polynomial?

I want to find the square root of a polynomial by the following process:

1. Compute the N-element DFT of its coefficients, maybe padded with zeros.
2. Compute the complex square root of each of the N elements of the DFT.
3. Take the inverse DFT of the resulting array.

For the time being, I am assuming that the polynomial is a perfect square, so that it will have a polynomial as its square root. I have checked that the foregoing process works fine in some cases.

My problem is that there are two choices (+/-) for each of the N square roots. Only if I make correct choices will I get the correct phase spectrum so that the IDFT will give me the correct polynomial as solution.

• How can I correctly choose the sign of each square root, other than (for example) by working through the 2^N possibilities until I find one that gives a polynomial as the result?

I have tried making use of the fact that the DFT should be 'smooth' if N is large relative to the order of the polynomial, but without getting anywhere.

[An example: if the polynomial has coefficients (1,2,3,4,3,2,1), its square root polynomial will have coefficients (1,1,1,1)]