  We present very simple non-integral relations between deep inelastic
structure functions $F_L$, $F_2$ and the gluon distribution at small $x$ based
on perturbative QCD which are useful for the phenomenological analysis of data
at low $x$. As an application we extract the deep inelastic scattering
cross-sections ratio $R= \sigma_L/\sigma_T$ in the range $10^{-4} \leq x \leq
10^{-2}$ from $F_2$ HERA data.
