Angular homeostasis IX. Polygonal orbits with a moving target: Implications for anomalous numbers of digits in congenital heart disease

This paper explores properties of discrete processes in which a pursuer seeks a target that is moving at constant velocity r that is a fixed proportion of the speed of the pursuer. The pursuer is subjected to proportional angular homeostasis, so chosen that the number of steps per circuit is small. The orbits relative to the target may assume any of four forms: polygons that reverse their sense an infinite number of times; or polygons that after a finite number of reversals ultimately come to have an integer numbers of sides; or have a rational numbers of sides; or have an irrational number of sides that densely fill an annulus. None of the polygons is regular. In the parameter space, the boundary line between the first of these sets and the other three has a somewhat bizarre pattern and may possibly be fractal, but no proof is forthcoming. Unlike the pattern with a stationary target, there may be a set or catchment of diverse values of the speed ratio, r, and the correction coefficient, b that all result in figures of some specified number, n, of sides (although with vertices in differing locations). Catchments have been found for only those polygons that have the winding number of 1. The implications are discussed that this property has for the genetic coding of biological traits that are countable. Some attention is also paid to the relevance of polygons with few sides to ontogenic growth when the correction coefficient is cyclically arc- or time-dependent. © 1993 Wiley-Liss, Inc.