I'm trying to understand the hypothesis of Marcinkiewicz-Mihlin-H"ormander multiplier theorem. See for instance Theorem A in following paper of Elias Stain.

Theorem A: Assumes that $m: (0, \infty)\to \mathbb R$ satisfies the following (1) $$|m^{(j)}(x)| \leq C x^{-j}$$ for $0 \leq j \leq k$ and $k>d/2.$ Or more generally

(2)

$$ \sup_{t>0} \left\|\chi m(t \cdot)\right\|_{L^2_{\alpha}}< \infty $$ where $\chi$ is a non-zero smooth cut-off function of compact support which vanishes near the origin.

Then Fourier-multiplier operator $\widehat{Tf}= m(|\xi|^2) \hat{f}$ is bounded on $L^p (\mathbb R^d)$ for $1<p< \infty.$

My questions are: (A) How to define $\|\cdot\|_{L^2_{\alpha}}$? Is it standard notation? (It seems that Stein has not defined this notation in his paper. Thus I guess it must be standard)

(B) Why condition (1) implies (2) in the Theorem?

(C) I'm interested to check symbol $m(\xi)=e^{i|\xi|^2}$ Satisfies the condition (2) of the Theorem A? (with this symbol we have solution to Schrodinger equations)

(D) How theorem has been developed historically? (I mean version of Marcinkiewicz theorem 1939, version of Mihim 1957, and finally version of Hormander 1960.)

My heuristic Guess is: we can define $\|f\|_{L^2_{\alpha}}^2= \int_{\mathbb R^d} |f(x)|^2 (1+|x|^2)^{\alpha} dx$ or $\|f\|_{L^2_{\alpha}}^2= \int_{\mathbb R^d} |\widehat{f}(\xi)|^2 (1+|\xi|^2)^{\alpha} dx$ Is this correct or am I missing something?