Inference in functional factor models with applications to yield curves

This paper develops a set of inferential methods for functional factor models that have been extensively used in modeling yield curves. Our setting accommodates both temporal dependence and heteroskedasticity. Firstly, we introduce an estimation approach based on minimizing the least squares loss function and establish the consistency and asymptotic normality of the estimators. Secondly, we propose a goodness-of-fit test that allows us to determine if a specific model fits the data. We derive the asymptotic distribution of the test statistics and this leads to a significance test. A simulation study establishes good finite sample performance of our inferential methods. An application to US and UK yield curves demonstrates the generality of our framework, which can accommodate both sparsely and densely observed yield curves

In vivo testing of crosslinked polyethers .2. Weight loss, IR analysis, and swelling behavior after implantation

As reported in Part I (''In vivo testing of crosslinked polyethers. I. Tissue reactions and biodegradation,'' I. Biomed. Mater. Xes., this issue, pp. 307-320), microscopical evaluation after implantation of crosslinked (co)polyethers in rats showed differences in the rate of biodegradation, depending on the presence of tertiary hydrogen atoms in the main chain and the hydrophilicity of the polyether system. Ln this article (Part II) the biostability will be discussed in terms of weight loss, the swelling behavior, and changes in the chemical structure of the crosslinked polyethers after implantation. The biostability increased in the order poly(POx) <poly(THF-co-OX) <poly(THF) for the relatively hydrophobic polyethers. This confirmed our hypothesis that the absence of tertiary hydrogen atoms would improve the biostability. On the other hand, signs of biodegradation were observed for all polyether system studied. Infrared surface analysis showed that biodegradation was triggered by oxidative attack on the polymeric chain, leading to the formation of carboxylic ester and acid groups. It also was found that in the THF-based (co)polyethers, alpha-methylene groups were more sensitive than beta-methylene groups. For a hydrophilic poly(THF)/PEO blend, an increase in surface PEO content was found, which might be due to preferential degradation of the PEO domains. (C) 1996 John Wiley & Sons, Inc