The pseudocircle is a finite topological space which is weakly homotopy equivalent to the standard circle.
In other word, for the purposes of homotopy theory (up to weak homotopy equivalence), it is equivalent to the circle, yet it is a purely finite combinatorial creature.
The pseudocircle is the topological space
whose underlying set is a -set, say (for the ‘left side’, ‘right side’, ‘top point’, and ‘bottom point’ of the circle)
and whose topological structure, given as the collection of open subsets, is
That is the topology generated by the base (which is in fact the unique minimal base and furthermore the unique minimal subbase).
These also name a partition of the standard circle, and this gives a quotient map? from the circle to the pseudocircle; this map is the promised weak homotopy equivalence, see below.
Relation to the standard circle
The function of sets
from the standard circle to the pseudocircle, which sends
a point in the open left half of (thought of under the standard embedding into ) to the point ;
a point in the open right half of to ;
the top point of to ;
the bottom point of to
is a continuous function. Moreover, it is a weak homotopy equivalence.
See the proof of the general statement at finite topological space - properties.
The only continuous functions in the other direction, are the constant maps.
- Michael McCord, Singular homology groups and homotopy groups of finite topological spaces, Duke Math. J. Volume 33, Number 3 (1966), 465-474. (EUCLID)
Revised on September 4, 2012 14:57:32
by Urs Schreiber