摘要:

For an abstract single‐stranded RNA, a combinatorial analysis is given for two important structures, hairpins and cloverleaves. The number of possible hairpins isO(2n), while the number of possible three‐petal cloverleaves isO(n32n), and the number of possible general cloverleaves isO((2.21

ISSN:0022-2526    

DOI:10.1002/sapm197960291

年代:1979

数据来源: WILEY

摘要:

The density wave theory of spiral structure in galaxies is studied as a dynamical problem: the gravitational instability of a galactic disk with respect to spiral modes. The asymptotic theory for tightly wound spirals is presented in detail, although the problem has been formulated in a more general form to open the way to future research work in other directions. Some preliminary results based on the general formulation are included; they confirm those based on the asymptotic theory in cases where the latter should be applicable. The fluid dynamical model is adopted; reference to the stellar dynamical model is made when necessary. Besides providing a coherent account of the present status of the theory, it is hoped that this article will further stimulate workers in continuum mechanics to play their important role in future research work in the astrophysics of galaxies.

ISSN:0022-2526    

DOI:10.1002/sapm197960297

年代:1979

数据来源: WILEY

摘要:

The graph of a partially ordered set (X, ⩽) hasXas its set of vertices and (x,y) is an edge if and only ifxcoversyorycoversx. The poset is path‐connected if its graph is connected. Two integer‐valued metrics, distance and fence, are defined for path‐connected posets. Together the values of these metrics determine a path‐connected poset to within isomorphism and duality. The result holds for path‐connected preordered sets where distance and fence are pseudometrics. The result fails for non‐path‐c

ISSN:0022-2526    

DOI:10.1002/sapm1979602165

年代:1979

数据来源: WILEY

摘要:

A trace evaluation is shown to supply an elementary proof of a weighted Burnside lemma for weights not necessarily constant on orbits.

ISSN:0022-2526    

DOI:10.1002/sapm1979602173

年代:1979

数据来源: WILEY

摘要:

Let us consider the one‐dimensional Schrödinger equation with the scattering potentialV(x)=2δ(x) and corresponding reflection coefficientb(k)=−i/(k+i). The potential satisfies a theorem of Deift and Trubowitz which states that non‐negative measurable potentialsV(x) satisfying a certain range condition have reflection coefficientsb(k) such thatb(0)=−1. We rescale the reflection coefficient forV(x)=2δ(x) by writingb(k)=−iv/(k + 1) where 0

ISSN:0022-2526    

DOI:10.1002/sapm1979602177

年代:1979

数据来源: WILEY