Strong coprimality and strong irreducibility of Alexander polynomials

A polynomial f(t) with rational coefficients is strongly irreducible if f(t^k) is irreducible for all positive integers k. Likewise, two polynomials f and g are strongly coprime if f(t^k) and g(t^l) are relatively prime for all positive integers k and l. We provide some sufficient conditions for strong irreducibility and prove that the Alexander polynomials of twist knots are pairwise strongly coprime and that most of them are strongly irreducible. We apply these results to describe the structure of the subgroup of the rational knot concordance group generated by the twist knots and to provide an explicit set of knots which represent linearly independent elements deep in the solvable filtration of the knot concordance group.Comment: 16 pages, 6 figure

What Can We Learn About the Sensitivity of Investment to Stock Prices with a Better Measure of Tobin's q?

This paper examines the responsiveness of investment to q (i.e., the ratio of a firm's market value to the replacement cost of its assets) using data on a unique type of firm: Real Estate Investment Trusts (REITs). For REITs, we have high quality estimates of the net asset value of the firm that we use to create relatively accurate measures of Tobin's q. In addition, REITs have institutional features that mitigate some of the complications faced by previous studies. We have three main results. First, there is little evidence of a statistical link between REIT investment and a traditional accounting-based measure of q. Second, REIT investment is highly sensitive to estimates of q that are based on analysts' appraisals of asset value. A REIT whose NAV-based q ratio rises from 1.0 to 1.1 will increase its assets by 4.3 percent in the next year. Third, the difference between the appraisal-based measure of q and the traditional accounting based measure typically increases with the age of the firm's assets and varies across types of properties. These results suggest that measurement error in q can lead to appreciable downward biases in investment sensitivities, even in an industry that seems to meet many of the assumptions in Tobin's original paper, but that Tobin's investment model performs well with a better measure of q.Investment; Tobin's q; Real Estate Investment Trusts

What Can We Learn About the Sensitivity of Investment to Stock Prices with a Better Measure of Tobin's q?

This paper examines the responsiveness of investment to q (i.e., the ratio of a firm's market value to the replacement cost of its assets) using data on a unique type of firm: Real Estate Investment Trusts (REITs). For REITs, we have high quality estimates of the net asset value of the firm that we use to create relatively accurate measures of Tobin's q. In addition, REITs have institutional features that mitigate some of the complications faced by previous studies. We have three main results. First, there is little evidence of a statistical link between REIT investment and a traditional accounting-based measure of q. Second, REIT investment is highly sensitive to estimates of q that are based on analysts' appraisals of asset value. A REIT whose NAV-based q ratio rises from 1.0 to 1.1 will increase its assets by 4.3 percent in the next year. Third, the difference between the appraisal-based measure of q and the traditional accounting based measure typically increases with the age of the firm's assets and varies across types of properties. These results suggest that measurement error in q can lead to appreciable downward biases in investment sensitivities, even in an industry that seems to meet many of the assumptions in Tobin's original paper, but that Tobin's investment model performs well with a better measure of q.Investment; Tobin's q; Real Estate Investment Trusts

Formation of ions and radicals from icy grains in comets

Ion and radical formation in comets are thought to occur primarily by photodissociation of gas phase molecules. Experimental evidence and theoretical calculations are presented that show that some of the radical and ions can come directly from ice grains. The experimental evidence suggest that if the frozen molecules on the surface of grains undergo direct dissociation then they may be able to release radicals directly in the gas phase. If the molecules undergo predissociation it is unlikely that they will release radicals in the gas phase since they should be quenched. Calculations of this direct photodissociation mechanism further indicate that even if the parent molecule undergoes direct dissociation, the yield will not be high enough to explain the rays structure in comets unless the radicals are stored in the grains and then released when the grain evaporates. Calculations were also performed to determine the maximum number of ions that can be stored in an icy grain's radius. This number is compared with the ratio of the ion to neutral molecular density. The comparison suggests that some of the ions observed near the nucleus of the comet could have originally been present in the cometary nucleus. It is also pointed out that the presence of these ions in icy grains could lead to radical formation via electron recombination. Finally, an avalanche process was evaluated as another means of producing ions in comets

Do Stock Prices Really Reflect Fundamental Values? The Case of REITs

Real estate investment trust (REIT) stock prices deviate substantially from net asset values (NAV). Using REIT data since 1990, we find large positive excess returns to a strategy of buying stocks that trade at a discount to NAV, and shorting stocks trading at a premium to NAV. Estimated alphas from this strategy are between 0.9% and 1.8% per month, with little risk. Trading costs and short-sale constraints are not prohibitive and the results strengthen when we control for differences in liquidity or the extent of institutional ownership. We find that some variation in P/NAV makes sense, as premiums are positively related to recent and future NAV growth. However, there appears to be too much volatility in P/NAV, giving rise to potential profits from short-term mean reversion. The closed-end fund literature has some similar findings on stock price deviations from fundamental value, but compared to closed-end funds REITs are much larger and have much higher insider and institutional ownership. These differences suggest that REIT premiums and discounts reflect more than just small investor sentiment, which is a common explanation of why closed-end fund prices deviate from their fundamental value.

The reactivity of [Pt₂(μ-S)₂(PPh₃)₄] towards difunctional chloroacetamide alkylating agents: Formation of cyclized or bridged products

The reactions of [Pt₂(μ-S)₂(PPh₃)₄] towards some bis(chloroacetamide) alkylating agents have been investigated. Reaction with one mole equivalent of the hydrazine-derived compound ClCH₂C(O)NHNHC(O)CH₂Cl led to the cyclized product [Pt₂{SCH₂C(O)NHNHC(O)CH₂S}(PPh₃)₄]²⁺ which showed two different PPh₃ environments in the ³¹P{1H} NMR spectrum, as a result of non-fluxional behavior of the dithiolate ligand in solution. Reactions of [Pt₂(μ-S)₂(PPh₃)₄] with the ortho and para isomers of the phenylenediamine-derived bis(chloroacetamides) ClCH2C(O)NHC6H4NHC(O)CH2Cl gave tetrametallic complexes containing two {Pt₂S₂} moieties spanned by the CH₂C(O)NHC₆H₄NHC(O)CH₂ group. Both the ortho and para isomers were crystallographically characterized; in the ortho isomer there is intramolecular CO=H–N and S•••H–N hydrogen bonding involving the two amide groups

Meteorological satellite products support for project COHMEX

The first year effort focussed on real-time support and satellite data collection during the field phase of COHMEX. Work efforts following the field phase of COHMEX concentrated on post-processing of the real-time data sets, and generation of enhanced, research-quality satellite data sets for selected COHMEX core days. These satellite-derived data sets will augment the special COHMEX conventional data base with high horizontal and temporal resolution information. The data sets will be examined for their usefulness in delineating important elements in the meteorological environment leading to convective activity. In addition, a limited research effort was conducted using the Cooperative Institute for Meteorological Satellite Studies (CIMSS) 4-d data assimilation system in conjunction with evaluating VISSR Atmospheric Sounder (VAS) and His-resolution Interferometer Sounder (HIS) data. The need to address the characteristics of the data types, and the problems they introduce into 4-d assimilation procedures is evident. The HIS instrument was flown aboard an ER-2 aircraft on several occasions during COHMEX. One of the flights was chosen for further study. Processed VAS soundings and COHMEX radiosonde data were also collected for this day. The case study included an evaluation of the HIS and VAS data and an impact study of the data on the assimilation system analysis