6,351 research outputs found
Stability estimates for the inverse boundary value problem by partial Cauchy data
In this paper we study the inverse conductivity problem with partial data in
dimension . We derive stability estimates for this inverse problem if
the conductivity has
regularity for
Modeling left-truncated and right-censored survival data with longitudinal covariates
There is a surge in medical follow-up studies that include longitudinal
covariates in the modeling of survival data. So far, the focus has been largely
on right-censored survival data. We consider survival data that are subject to
both left truncation and right censoring. Left truncation is well known to
produce biased sample. The sampling bias issue has been resolved in the
literature for the case which involves baseline or time-varying covariates that
are observable. The problem remains open, however, for the important case where
longitudinal covariates are present in survival models. A joint likelihood
approach has been shown in the literature to provide an effective way to
overcome those difficulties for right-censored data, but this approach faces
substantial additional challenges in the presence of left truncation. Here we
thus propose an alternative likelihood to overcome these difficulties and show
that the regression coefficient in the survival component can be estimated
unbiasedly and efficiently. Issues about the bias for the longitudinal
component are discussed. The new approach is illustrated numerically through
simulations and data from a multi-center AIDS cohort study.Comment: Published in at http://dx.doi.org/10.1214/12-AOS996 the Annals of
Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical
Statistics (http://www.imstat.org
Storage Size Determination for Grid-Connected Photovoltaic Systems
In this paper, we study the problem of determining the size of battery
storage used in grid-connected photovoltaic (PV) systems. In our setting,
electricity is generated from PV and is used to supply the demand from loads.
Excess electricity generated from the PV can be stored in a battery to be used
later on, and electricity must be purchased from the electric grid if the PV
generation and battery discharging cannot meet the demand. Due to the
time-of-use electricity pricing, electricity can also be purchased from the
grid when the price is low, and be sold back to the grid when the price is
high. The objective is to minimize the cost associated with purchasing from (or
selling back to) the electric grid and the battery capacity loss while at the
same time satisfying the load and reducing the peak electricity purchase from
the grid. Essentially, the objective function depends on the chosen battery
size. We want to find a unique critical value (denoted as ) of the
battery size such that the total cost remains the same if the battery size is
larger than or equal to , and the cost is strictly larger if the
battery size is smaller than . We obtain a criterion for evaluating
the economic value of batteries compared to purchasing electricity from the
grid, propose lower and upper bounds on , and introduce an efficient
algorithm for calculating its value; these results are validated via
simulations.Comment: Submitted to IEEE Transactions on Sustainable Energy, June 2011; Jan
2012 (revision
- …