How can I represent the following equation in matrix form?
$$|a x + b y| \leq k_1$$ $$|c x + d y| \leq k_2$$
where a, b, c and d are complex. I am confused with the absolute sign since its not a linear operator. I could use Cauchy Schwarz inequality but that would not exactly fulfill the inequality conditions.
Edit: $x, y, k_1, k_2 \in \mathbb{R}$