  An interpolating function $\tilde F$ between the $a$-anomaly coefficient in
even dimensions and the free energy on an odd-dimensional sphere has been
proposed recently and is conjectured to monotonically decrease along any
renormalization group flow in continuous dimension $d$. We examine $\tilde F$
in the large-$N$ CFT's in $d$ dimensions holographically described by the
Einstein-Hilbert gravity in the AdS$_{d+1}$ space. We show that $\tilde F$ is a
smooth function of $d$ and correctly interpolates the $a$ coefficients and the
free energies. The monotonicity of $\tilde F$ along an RG flow follows from the
analytic continuation of the holographic $c$-theorem to continuous $d$, which
completes the proof of the conjecture.
