= Solution
Under the stated approximation the perturbation equation is
$$
\ddot\delta+2H\dot\delta=0,
\qquad
\dot\delta\propto a^{-2}.
$$
During <radiation domination>, $a\propto t^{1/2}$, so $\dot\delta\propto t^{-1}$ and
$$
\delta=C_1+C_2\log t=C_1+2C_2\log a.
$$
Matter perturbations therefore have only <logarithmic growth of matter perturbations during radiation domination>.
In a <curvature-dominated universe>, $a\propto t$, so $\dot\delta\propto t^{-2}$ and
$$
\delta=C_1+C_2t^{-1}=C_1+C_2a^{-1}.
$$
The nondecaying mode is constant: curvature domination gives <freezing of matter perturbations during curvature domination>, rather than continued growth.
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