Journal of Formalized Mathematics
Volume 11, 1999
University of Bialystok
Copyright (c) 1999 Association of Mizar Users

## Rotating and Reversing

Andrzej Trybulec
University of Bialystok

### Summary.

Quite a number of lemmas for the Jordan curve theorem, as yet in the case of the special polygonal curves, have been proved. By special" we mean, that it is a polygonal curve with edges parallel to axes and actually the lemmas have been proved, mostly, for the triangulations i.e. for finite sequences that define the curve. Moreover some of the results deal only with a special case: \begin{itemize} \item[-] finite sequences are clockwise oriented, \item[-] the first member of the sequence is the member with the lowest ordinate among those with the highest abscissa (N-min $f,$ where $f$ is a finite sequence, in the Mizar jargon). \end{itemize} In the change of the orientation one has to reverse the sequence (the operation introduced in [6]) and to change the second restriction one has to rotate the sequence (the operation introduced in [19]). The goal of the paper is to prove, mostly simple, facts about the relationship between properties and attributes of the finite sequence and its rotation (similar results about reversing had been proved in [6]). Some of them deal with recounting parameters, others with properties that are invariant under the rotation. We prove also that the finite sequence is either clockwise oriented or it is such after reversing. Everything is proved for the so called standard finite sequences, which means that if a point belongs to it then every point with the same abscissa or with the same ordinate, that belongs to the polygon, belongs also to the finite sequence. It does not seem that this requirement causes serious technical obstacles.

#### MML Identifier: REVROT_1

The terminology and notation used in this paper have been introduced in the following articles [17] [22] [2] [15] [1] [4] [3] [5] [9] [21] [10] [6] [19] [16] [7] [8] [11] [12] [13] [18] [20] [14]

#### Contents (PDF format)

1. Preliminaries
3. Rotating Circular Ones
4. Finite Sequence on the Plane
5. Rotating Finite Sequence on the Plane
6. Rotating Circular Ones (on the Plane)
7. The Orientation

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