Units of Q of square root two (${\mathcal{O}}_{\mathbb{Q}(\sqrt{2})}^{\times }$)

The unit $\epsilon =1+\sqrt{2}$ has norm minus one and is the least unit greater than one in the positive real embedding. Dirichlet unit theorem therefore gives

Reduction modulo $(3+\sqrt{2})$ identifies $\mathbb{Z}[\sqrt{2}]/(3+\sqrt{2})$ with ${\mathbb{F}}_7$, sends $\sqrt{2}$ to $4$, and sends $\epsilon =1+\sqrt{2}$ to $5$. Since $5$ has order six and $5^3=-1$ in ${\mathbb{F}}_7^{\times }$, the kernel on units is the infinite cyclic group