When searching here to find methods for solving quartic polynomials, I came across a question where one of the solutions (at the very end) mentions that the equating coefficients method can fail. https://math.stackexchange.com/a/57688
The example given is $$x^4−x−1=0$$ and indeed when equating coefficients of $$x^4−x−1=(x^2+bx+c)(x^2+Bx+C)$$ the resulting system is inconsistent.
I've not been able to find a formal statement or proof of the equating the coefficients theorem to try to figure out if some assumption has been violated or why it doesn't work in this case.
I don't have the background yet to understand an explanation based on the Galois groups that are briefly mentioned in the link, so if someone could find another way to explain it, that would be appreciated. An additional explanation based on Galois groups may also help someone else though.
https://en.wikipedia.org/wiki/Equating_coefficients doesn't mention anything about where it might fail.