Are there any known inequalities of the following type for $f$ satisfying some conditions: $$ \|f\|_{H_p(\mathbb{R})} \le C\|f\|_{L_p(\mathbb{R})} $$ ? Of particular interest to me is the following case: Assume that $\varphi$ satisfies the inequality above. Is the same true for any function in the integer-shift-invariant space spanned by the integer shifts of $\varphi$?