Input Diffusion and the Evolution of Production Networks

The adoption and diffusion of inputs in the production network is at the heart of technological progress. What determines which inputs are initially considered and eventually adopted by innovators? We examine the evolution of input linkages from a network perspective, starting from a stylized model of network formation. Producers direct their search for new inputs along vertical linkages, screening the network neighborhood of existing suppliers to identify potentially useful inputs. A subset of these is then adopted, following a tradeoff between the benefits from input variety and the costs of customizing new inputs. Guided by this framework, we document a novel stylized fact at both the sector and the firm level: producers are more likely to adopt inputs that are already used – directly or indirectly – by their current suppliers. In particular, using disaggregated input-output data, we show that initial network proximity of a sector in 1967 significantly increases the likelihood of adoption throughout the subsequent four decades. A one-standard deviation decrease in network distance is associated with an increase in the adoption probability by one third to one half. Similarly, U.S. firms are significantly more likely to develop new input linkages among their suppliers' network neighborhood. Our results imply that the existing production network plays a crucial role in the diffusion of inputs and the evolution of technology

Input Diffusion and the Evolution of Production Networks

The adoption and diffusion of inputs in the production network is at the heart of technological progress. What determines which inputs are initially considered and eventually adopted by innovators? We examine the evolution of input linkages from a network perspective, starting from a stylized model of network formation. Producers direct their search for new inputs along vertical linkages, screening the network neighborhood of existing suppliers to identify potentially useful inputs. A subset of these is then adopted, following a tradeoff between the benefits from input variety and the costs of customizing new inputs. Guided by this framework, we document a novel stylized fact at both the sector and the firm level: producers are more likely to adopt inputs that are already used – directly or indirectly – by their current suppliers. In particular, using disaggregated input-output data, we show that initial network proximity of a sector in 1967 significantly increases the likelihood of adoption throughout the subsequent four decades. A one-standard deviation decrease in network distance is associated with an increase in the adoption probability by one third to one half. Similarly, U.S. firms are significantly more likely to develop new input linkages among their suppliers' network neighborhood. Our results imply that the existing production network plays a crucial role in the diffusion of inputs and the evolution of technology

Blue rubber-bleb nevus syndrome: report of a familial case with a dural arteriovenous fistula

We report a case of a 45-year-old woman known to have a familial form of blue rubber-bleb nevus syndrome (BRBNS). The patient developed severe occipital headaches and bilateral retroauricular bruits. Cerebral angiography showed a large dural arteriovenous fistula in the torcular region. Central nervous system involvement in BRBNS is not often reported, and most cases of BRBNS are sporadic

Toward a test of angular momentum coherence in a twin-atom interferometer

We present a scheme well-suited to investigate quantitatively the angular momentum coherence of molecular fragments. Assuming that the dissociated molecule has a null total angular momentum, we investigate the propagation of the corresponding atomic fragments in the apparatus. We show that the envisioned interferometer enables one to distinguish unambiguously a spin-coherent from a spin-incoherent dissociation, as well as to estimate the purity of the angular momentum density matrix associated with the fragments. This setup, which may be seen as an atomic analogue of a twin-photon interferometer, can be used to investigate the suitability of molecule dissociation processes -- such as the metastable hydrogen atoms H($2^2 S$)-H($2^2 S$) dissociation - for coherent twin-atom optics.Comment: 6 pages, 3 Figures. Final version accepted for publication in Europhysics Letter

A mathematical framework for reducing the domain in the mechanical analysis of periodic structures

A theoretical framework is developped leading to a sound derivation of Periodic Boundary Conditions (PBCs) for the analysis of domains smaller then the Unit Cells (UCs), named reduced Unit Cells (rUCs), by exploiting non-orthogonal translations and symmetries. A particular type of UCs, Offset-reduced Unit Cells (OrUCs) are highlighted. These enable the reduction of the analysis domain of the traditionally defined UCs without any loading restriction. The relevance of the framework and its application to any periodic structure is illustrated through two practical examples: 3D woven and honeycomb.Comment: 18 page

Self-organized patterns of coexistence out of a predator-prey cellular automaton

We present a stochastic approach to modeling the dynamics of coexistence of prey and predator populations. It is assumed that the space of coexistence is explicitly subdivided in a grid of cells. Each cell can be occupied by only one individual of each species or can be empty. The system evolves in time according to a probabilistic cellular automaton composed by a set of local rules which describe interactions between species individuals and mimic the process of birth, death and predation. By performing computational simulations, we found that, depending on the values of the parameters of the model, the following states can be reached: a prey absorbing state and active states of two types. In one of them both species coexist in a stationary regime with population densities constant in time. The other kind of active state is characterized by local coupled time oscillations of prey and predator populations. We focus on the self-organized structures arising from spatio-temporal dynamics of the coexistence. We identify distinct spatial patterns of prey and predators and verify that they are intimally connected to the time coexistence behavior of the species. The occurrence of a prey percolating cluster on the spatial patterns of the active states is also examined.Comment: 19 pages, 11 figure