An interval map has a horseshoe for an interval map when two disjoint subintervals are each mapped across a common interval. It is Glendinning-chaotic when one of its positive iterates has such a horseshoe.
Let the points of the three-cycle be and put , . There are two possible cyclic orders. If , the intermediate value theorem gives
for the interval covering relation. Thus there are two closed covering walks of length two based at . In the reverse cyclic order there are instead the arrows , and , again giving two closed walks of length two. The horseshoe from two closed covering walks therefore shows that has a horseshoe in either case, so is chaotic.
The Sharkovsky theorem orders the positive integers as
and says that a cycle of a given period forces cycles of every period to its right.
Suppose with odd. Then has a -cycle. The Sharkovsky theorem gives that map a six-cycle, and squaring it produces a three-cycle. Hence has a three-cycle and an iterate of has a horseshoe. Thus
A horseshoe contains the symbolic dynamics of the Bernoulli shift. Periodic binary words give cycles of every symbolic period; after translating from an iterate back to , this supplies cycles of many periods that are not powers of two.
For the logistic map, a ten-cycle would imply chaos because is not a power of two. Hence there is no ten-cycle for . A three-cycle forces every period, so a ten-cycle exists for . The stated facts alone give no conclusion for the intermediate range: