Past exam of the mathematics course of the University of Oxford / 2021 / Prelims / Mathematics I / 2 / b / ii / Solution

Suppose, for a contradiction, that $p$ and $q$ have no common root of a polynomial. Since the polynomial ring $\mathbb{C}[x]$ has no zero divisors and $p,q\ne 0$, the relation $ap+bq=0$ with $a,b$ not both zero forces both $a$ and $b$ to be nonzero.

By the fundamental theorem of algebra, writewhere the ${\alpha }_j$ are distinct. For each $j$, $q({\alpha }_j)\ne 0$. The equality $bq=-ap$ implies that $(x-{\alpha }_j{)}^{m_j}$ divides $bq$. The multiplicity of a root in polynomial multiplication is the sum of its multiplicities in the factors: factor out the largest powers of $x-{\alpha }_j$ and note that the two remaining factors have nonzero values at ${\alpha }_j$. Since $q({\alpha }_j)\ne 0$, $b$ must contain $(x-{\alpha }_j{)}^{m_j}$.

Thus $b$ has at least ${\sum }_j{m}_j=n$ roots of a polynomial counted with multiplicity of a root, so $\mathrm{deg}b\ge n$. This contradicts $b\in V_{n-1}$. ConsequentlyCounting multiplicity of a root is necessary: counting only distinct roots of a polynomial would miss repeated factors.