منابع مشابه
Perfect time or perfect crime?
Imagine the situation: “One day, you notice an old country woman crossing a downtown street, just about to put one foot down on the rails of the streetcar line. [...] Suppose, now, that at the very moment she puts her foot on the rail a streetcar comes rushing down the tracks toward her. If the old woman does not notice the car and continues across the tracks nothing will happen. But if someone...
متن کاملPerfect Matchings and Perfect Powers
In the last decade there have been many results about special families of graphs whose number of perfect matchings is given by perfect or near perfect powers (N. Elkies et al., J. Algebraic Combin. 1 (1992), 111– 132; B.-Y. Yang, Ph.D. thesis, Department of Mathematics, MIT, Cambridge, MA, 1991; J. Propp, New Perspectives in Geometric Combinatorics, Cambridge University Press, 1999). In this pa...
متن کاملPerfect Matchings and Perfect Squares
In 1961, P.W. Kasteleyn enumerated the domino tilings of a 2n × 2n chessboard. His answer was always a square or double a square (we call such a number "squarish"), but he did not provide a combinatorial explanation for this. In the present thesis, we prove by mostly combinatorial arguments that the number of matchings of a large class of graphs with 4-fold rotational symmetry is squarish; our ...
متن کاملOn generalizations of semiperfect and perfect rings
We call a ring $R$ right generalized semiperfect if every simple right $R$-module is an epimorphic image of a flat right $R$-module with small kernel, that is, every simple right $R$-module has a flat $B$-cover. We give some properties of such rings along with examples. We introduce flat strong covers as flat covers which are also flat $B$-covers and give characterizations of $A$-perfe...
متن کاملPerfect $2$-colorings of the Platonic graphs
In this paper, we enumerate the parameter matrices of all perfect $2$-colorings of the Platonic graphs consisting of the tetrahedral graph, the cubical graph, the octahedral graph, the dodecahedral graph, and the icosahedral graph.
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ژورنال
عنوان ژورنال: Microscopy Today
سال: 1999
ISSN: 1551-9295,2150-3583
DOI: 10.1017/s1551929500064105