Ubiquity of synonymity: almost all large binary trees are not uniquely identified by their spectra or their immanantal polynomials

There are several common ways to encode a tree as a matrix, such as the adjacency matrix, the Laplacian matrix (that is, the infinitesimal generator of the natural random walk), and the matrix of pairwise distances between leaves. Such representations involve a specific labeling of the vertices or at least the leaves, and so it is natural to attempt to identify trees by some feature of the associated matrices that is invariant under relabeling. An obvious candidate is the spectrum of eigenvalues (or, equivalently, the characteristic polynomial). We show for any of these choices of matrix that the fraction of binary trees with a unique spectrum goes to zero as the number of leaves goes to infinity. We investigate the rate of convergence of the above fraction to zero using numerical methods. For the adjacency and Laplacian matrices, we show that that the {\em a priori} more informative immanantal polynomials have no greater power to distinguish between trees

The Yield Curve Slope and Monetary Policy Innovations

We separate changes of the federal funds rate into two components; one reflects the Fed's superior forecasts about the state of the economy and the other component reflects the Fed's reaction to the public's forecast about the state of the economy. Romer and Romer (2000) found that the Fed reveals information about inflation when it tightens monetary policy. Their research has implications for measuring monetary policy as well. When the Fed raises short-term interest rates it leads to some combination of increased inflationary expectations and an increased real rate. In this paper we estimate a structural VAR that allows us to separate out (identify) components of federal funds changes that are due to inflationary expectations (thus neutral) and that part which is contractionary. Our measure of monetary policy is the part of federal funds changes that exclude the Fed's revelation of its asymmetric information about future inflation.Monetary policy, Yield curve, Inflation, Price puzzle

Development of an innovative method for the evaluation of fungal contamination of surfaces

The objective of this technical report is to compare the ability to capture fungal spores through samples performed with three different methods: Rodac contact plates, cotton pad and a pad prepared with a dusting cloth (DC pads) selected from those available on the market. The tests were conducted using a suspension of Aspergillus niger conidia equal to 0.5 MacFarland diluted 1/30, 1/40, 1/50, 1/100. With each of these dilutions 3 sterile tiles of stainless steel were contaminated, each divided into 16 small squares, in the center of which 0.1 ml of the dilution chosen was placed and left to dry (for a total of 12 sheets). In addition, we have used other 6 tiles to repeat the experience with dilutions 1/40 and 1/50. A total of 288 squared surfaces were contaminated: 96 of these were sampled with Rodac contact plates, 96 with cotton pads and 96 with DC and then inseminated in Petri plates. Sabouraud dextrose agar was used as culture medium for the first 12 plates, while, for the other 6 plates Sabouraud dextrose agar added with lecithin and polysorbate 80 was used. All plates were incubated at 37 degrees for 18 hours. To estimate the differences among the sampling methods and the dilutions tested, multiple linear regression was used. The analysis showed that the number of colonies harvested at dilution 1/40 is 13% higher (P = 0.09) than the number harvested at dilution 1/50 and the number of colonies harvested at dilution 1/30 is 6% higher than dilution 1/50 (P = 0.52). With regard to the comparison between the number of colonies harvested with Rodac contact plates, with cotton pads and DC pads, regression analysis shows that cotton pads harvest a number of fungal cfu 5 times higher than those detected with Rodac plates, while DC pads harvest a number of fungal ufc 6 times higher than those detected with Rodac plates (P < 0.00005). These results, although preliminary, indicate that DC pads are a sensitive and simple approach for the environmental control of fungal contamination

Earth orbital teleoperator visual system evaluation program

Visual system parameters and stereoptic television component geometries were evaluated for optimum viewing. The accuracy of operator range estimation using a Fresnell stereo television system with a three dimensional cursor was examined. An operator's ability to align three dimensional targets using vidicon tube and solid state television cameras as part of a Fresnell stereoptic system was evaluated. An operator's ability to discriminate between varied color samples viewed with a color television system was determined

STEM CELL GROWTH AND DIFFERENTIATION IN HYDRA ATTENUATA

The differentiation of nerve cells and nematocytes from interstitial stem cells in Hydra has been investigated under conditions of changing stem cell density. Interstitial stem cells were cultured in a feeder layer system consisting of aggregates of nitrogen mustard-inactivated tissue. The aggregates were seeded with varying numbers of stem cells from 10 to 400 per aggregate; between 4 and 7 days later the rates of nerve and nematocyte differentiation were measured. Nerve differentiation was scored by labelling the stem cell population with [3H]-thymidine and counting nests of 4 proliferating nematoblasts. In both cases the numbers of differentiating cells were normalized to the size of the stem cell population. The results indicate that the rate of nematocyte differentiation increases as the concentration of stem cells increases in aggregates; under the same conditions the rate of nerve differentiation remains essentially constant. To calculate the numbers of stem cells entering each pathway per generation, a computer was programmed to simulate the growth and differentiation of interstitial stem cells. Standard curves were prepared from the simulations relating the rates of nerve and nematocyte differentiation to the fraction of stem cells committed to each pathway per generation. The rates of nerve and nematocyte commitment were then estimated from the experimentally observed rates of differentiation using the standard curves. The results indicate that nerve commitment remains constant at about 0.13 stem cells per generation over a wide range of stem cell concentration. Nematocyte commitment, by comparison, increases from 0.15 to 0.21 stem cells per generation as stem cell concentration increases in aggregates. The fact that the ratio of nerve to nematocyte commitment changes under our conditions suggests that stem cell commitment is not a stochastic process but subject to control by environmental stimuli

Survival analysis of DNA mutation motifs with penalized proportional hazards

Antibodies, an essential part of our immune system, develop through an intricate process to bind a wide array of pathogens. This process involves randomly mutating DNA sequences encoding these antibodies to find variants with improved binding, though mutations are not distributed uniformly across sequence sites. Immunologists observe this nonuniformity to be consistent with "mutation motifs", which are short DNA subsequences that affect how likely a given site is to experience a mutation. Quantifying the effect of motifs on mutation rates is challenging: a large number of possible motifs makes this statistical problem high dimensional, while the unobserved history of the mutation process leads to a nontrivial missing data problem. We introduce an $\ell_1$-penalized proportional hazards model to infer mutation motifs and their effects. In order to estimate model parameters, our method uses a Monte Carlo EM algorithm to marginalize over the unknown ordering of mutations. We show that our method performs better on simulated data compared to current methods and leads to more parsimonious models. The application of proportional hazards to mutation processes is, to our knowledge, novel and formalizes the current methods in a statistical framework that can be easily extended to analyze the effect of other biological features on mutation rates