= Solution
The <AdS scalar-field boundary asymptotics> are
$$
\phi(x,z)=z^{3-\Delta}J(x)+z^\Delta A(x)+\cdots
=zJ(x)+z^2A(x)+\cdots.
$$
Thus the <holographic dictionary> becomes especially simple for the flat field $\psi=\phi/z$:
$$
\boxed{J(x)=\lim_{z\to0}\frac{\phi(x,z)}z
=\psi(x,0)},
$$
$$
\boxed{\langle\mathcal O(x)\rangle
=(2\Delta-d)A(x)=A(x)
=\lim_{z\to0}\partial_z\left(\frac{\phi(x,z)}z\right)}
$$
up to the overall normalization chosen for the bulk action and possible local counterterms. The leading mode is the source and the normalizable subleading mode is the response.
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