We prove uniqueness of the maximum likelihood estimator for the class of
k-monotone densities.
We study estimation of multivariate densities $p$ of the form $p(x) =
h(g(x))$ for $x \in R^d$ and for a fixed function $h$ and an unknown convex
function $g$. The canonical example is $h(y) = e^{-y}$ for $y \in R$; in this
case the resulting class of densities $$\mathcal{P}(e^{-y}) = \{p = \exp(-g) :
g is convex \}$$ is well-known as the class of log-concave densities. Other
functions $h$ allow for classes of classes of densities with heavier tails than
the log-concave class.
Inference problems with incomplete observations often aim at estimating
population-level properties of complete data. We introduce a simple empirical
correction that provides partial protection against model misspecification
during such estimation. Unlike nonparametric or semiparametric techniques, our
empirical correction does not produce consistent estimates. Instead, our method
first fits a misspecified parametric model, whose plug-in estimate of the
quantity of interest is naturally inconsistent.
We establish limit theory for the Grenander estimator of a monotone density
near zero. In particular we consider the situation when the true density $f_0$
is unbounded at zero, with different rates of growth to infinity. In the course
of our study we develop new switching relations by use of tools from convex
analysis. The theory is applied to a problem involving mixtures.