On biquadratic fields: when 5 squares are not enough

In this paper we study the Pythagoras number $\mathcal{P}({\mathcal{O}}_K)$ for the rings of integers in totally real biquadratic fields $K$. We continue the work of Tinková towards proving the conjecture by Krásenský, Raška and Sgallová that a biquadratic $K$ satisfies $\mathcal{P}({\mathcal{O}}_K)\ge 6$ if and only if it contains neither $\sqrt{2}$ nor $\sqrt{5}$, with only finitely many exceptions. We fully solve two out of three remaining classes of fields by proving that all but finitely many $K$ containing $\sqrt{6}$ or $\sqrt{7}$ satisfy $\mathcal{P}({\mathcal{O}}_K)\ge 6$. Furthermore, we present ideas and computations which further support the conjecture also for $K$ containing $\sqrt{3}$. This enables us to refine the conjecture by explicitly listing the exceptional fields.