  We study the problem of selling a divisible item to agents who have concave
valuation functions for fractions of the item. This is a fundamental problem
with apparent applications to pricing communication bandwidth or cloud
computing services. We focus on simple sequential posted pricing mechanisms
that use linear pricing, i.e., a fixed price for the whole item and
proportional prices for fractions of it. We present results of the following
form that can be thought of as analogs of the well-known prophet inequality of
Samuel-Cahn (1984). For $\rho\approx 32\%$, if there is a linear pricing so
that sequential posted pricing sells a $\rho$-fraction of the item, this
results in a $\rho$-approximation of the optimal social welfare. The value of
$\rho$ can be improved to approximately $42\%$ if sequential posted pricing
considers the agents in random order. We also show that the best linear pricing
yields an expected revenue that is at most $O(\kappa^2)$ times smaller than the
optimal one, where $\kappa$ is a bound on the curvature of the valuation
functions. The proof extends and exploits the approach of Alaei et al. (2019)
and bounds the revenue gap by the objective value of a mathematical program.
The dependence of the revenue gap on $\kappa$ is unavoidable as a lower bound
of $\Omega(\ln{\kappa})$ indicates.
