Roth-type theorems in $K_{s,t}$-free sets

We show that for all integers $2\le s\le t$, any $K_{s,t}$-free subset of $[N]$ with size $\mathrm{\Omega }(n^{1-1\mathrm{/}s})$ must contain a nontrivial solution to every fixed translation-invariant linear equation in at least five variables. This extends earlier results for Sidon sets due to Conlon-Fox-Sudakov-Zhao and Prendiville to the full family of $K_{s,t}$-free sets. We also study the corresponding problem in vector spaces over finite fields. In ${\mathbb{F}}_q^n$ we obtain stronger quantitative bounds, including polylogarithmic savings, by combining Fourier-analytic transference with polynomial-method input from the arithmetic cycle-removal lemma of Fox-Lovász-Sauermann.