Published online by Cambridge University Press: 20 November 2018
Let ${{\mathbf{F}}_{q}}[T]$ be the ring of polynomials over the finite field of
$q$ elements and
$Y$ a large integer. We say a polynomial in
${{\mathbf{F}}_{q}}[T]$ is
$Y$-smooth if all of its irreducible factors are of degree at most
$Y$. We show that a ternary additive equation
$a\,+\,b\,=\,c$ over
$Y$-smooth polynomials has many solutions. As an application, if
$S$ is the set of first
$s$ primes in
${{\mathbf{F}}_{q}}[T]$ and
$s$ is large, we prove that the
$S$-unit equation
$u\,+\,v\,=\,1$ has at least
$\text{exp}\left( {{s}^{1/6-\in }}\,\text{log}\,\text{q} \right)$ solutions.