Scaling necessity for an Lp to Lq Fourier bound
= Scaling necessity for an Lp to Lq Fourier bound
If a nontrivial uniform estimate
$$
\|\widehat f\|_q\leq C\|f\|_p
$$
holds for all integrable $L^p$ functions on $\mathbb R^n$, dilation by every $\lambda>0$ forces
$$
1-\frac1q=\frac1p.
$$
Thus $q$ must be the <conjugate exponents>[conjugate exponent] of $p$.