Why Do Computers Depreciate?

The value of installed computers falls rapidly and therefore computers have a very high user cost. The paper provides a complete account of the non-financial user cost of personal computers -- decomposing it into replacement cost change, obsolescence, instantaneous depreciation, and age-related depreciation. The paper uses data on the resale price of computers and a hedonic price index for new computers to achieve this decomposition. Once obsolescence is taken into account, age-related depreciation -- which is often identified as deterioration -- is estimated to be negligible. While the majority of the loss in value of used computers comes from declines in replacement cost, this paper shows the second most important source of decline in value is obsolescence. Obsolescence is accelerated by the decline in replacement cost of computers. Cheaper computing power drives developments in software and networks that make older computers less productive even though their original functionality remains intact.

Automatic structures, rational growth and geometrically finite hyperbolic groups

We show that the set $SA(G)$ of equivalence classes of synchronously automatic structures on a geometrically finite hyperbolic group $G$ is dense in the product of the sets $SA(P)$ over all maximal parabolic subgroups $P$. The set $BSA(G)$ of equivalence classes of biautomatic structures on $G$ is isomorphic to the product of the sets $BSA(P)$ over the cusps (conjugacy classes of maximal parabolic subgroups) of $G$. Each maximal parabolic $P$ is a virtually abelian group, so $SA(P)$ and $BSA(P)$ were computed in ``Equivalent automatic structures and their boundaries'' by M.Shapiro and W.Neumann, Intern. J. of Alg. Comp. 2 (1992) We show that any geometrically finite hyperbolic group has a generating set for which the full language of geodesics for $G$ is regular. Moreover, the growth function of $G$ with respect to this generating set is rational. We also determine which automatic structures on such a group are equivalent to geodesic ones. Not all are, though all biautomatic structures are.Comment: Plain Tex, 26 pages, no figure

Genetic architecture of parallel armor plate reduction in threespine sticklebacks

Journal ArticleHow many genetic changes control the evolution of new traits in natural populations? Are the same genetic changes seen in cases of parallel evolution? Despite long-standing interest in these questions, they have been difficult to address, particularly in vertebrates. We have analyzed the genetic basis of natural variation in three different aspects of the skeletal armor of threespine sticklebacks (Gasterosteus aculeatus): the pattern, number, and size of the bony lateral plates. A few chromosomal regions can account for variation in all three aspects of the lateral plates, with one major locus contributing to most of the variation in lateral plate pattern and number. Genetic mapping and allelic complementation experiments show that the same major locus is responsible for the parallel evolution of armor plate reduction in two widely separated populations. These results suggest that a small number of genetic changes can produce major skeletal alterations in natural populations and that the same major locus is used repeatedly when similar traits evolve in different locations

Cynodont from the Upper Triassic of East Greenland: tooth replacement and double-rootedness

Journal ArticleA new genus and species of cynodont from the Upper Triassic Fleming Fjord Formation of East Greenland possesses double-rooted postcanine teeth and a nonalternate pattern of tooth replacement. The specimen represents an addition to the known diversity of Early Mesozoic taxa with multi-rooted dentitions (tritylodontids, Sinoconodon sp., haramiyids, morganucodontids, Meurthodon gallicus), and casts doubt on traditional interpretations of the interdependency of reduced tooth replacement patterns and teeth with multiple roots

Limb diversity and digit reduction in reptilian evolution

Journal ArticleThe study of morphological rules, or trends, offered classical biologists the opportunity to address the mechanisms underlying the evolution of anatomical designs. Regularities in evolution suggested that common functional or developmental rules governed the transformation of structures. Parallelism is one such example

Divergence, convergence, and the ancestry of feral populations in the domestic rock pigeon

ManuscriptDomestic pigeons are spectacularly diverse and exhibit variation in more traits than any other bird species [1]. In The Origin of Species, Charles Darwin repeatedly calls attention to the striking variation among domestic pigeon breeds - generated by thousands of years of artificial selection on a single species by human breeders - as a model for the process of natural divergence among wild populations and species [2]. Darwin proposed a morphology-based classification of domestic pigeon breeds [3], but the relationships among major groups of breeds and their geographic origins remain poorly understood [4, 5]. We used a large, geographically diverse sample of 361 individuals from 70 domestic pigeon breeds and two free-living populations to determine genetic relationships within this species. We found unexpected relationships among phenotypically divergent breeds that imply convergent evolution of derived traits in several breed groups. Our findings also illuminate the geographic origins of breed groups in India and the Middle East, and suggest that racing breeds have made substantial contributions to feral pigeon populations