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where the left-hand side denotes the i-th homotopy group.
The requirements of being non-empty and path-connected can be interpreted as (−1)-connected and 0-connected, respectively, which is useful in defining 0-connected and 1-connected maps, as below. The 0-th homotopy set can be defined as:
This is only a pointed set, not a group, unless X is itself a topological group; the distinguished point is the class of the trivial map, sending S0 to the base point of X. Using this set, a space is 0-connected if and only if the 0th homotopy set is the one-point set. The definition of homotopy groups and this homotopy set require that X be pointed (have a chosen base point), which cannot be done if X is empty.
A topological space X is path-connected if and only if its 0-th homotopy group vanishes identically, as path-connectedness implies that any two points x1 and x2 in X can be connected with a continuous path which starts in x1 and ends in x2, which is equivalent to the assertion that every mapping from S0 (a discrete set of two points) to X can be deformed continuously to a constant map. With this definition, we can define X to be n-connected if and only if
- A space X is (−1)-connected if and only if it is non-empty.
- A space X is 0-connected if and only if it is non-empty and path-connected.
- A space is 1-connected if and only if it is simply connected.
Thus, the term "n-connected" is a natural generalization of being non-empty, path-connected, or simply connected.
It is obvious from the definition that an n-connected space X is also i-connected for all i < n.
The corresponding relative notion to the absolute notion of an n-connected space is an n-connected map, which is defined as a map whose homotopy fiber Ff is an (n − 1)-connected space. In terms of homotopy groups, it means that a map is n-connected if and only if:
- is an isomorphism for , and
- is a surjection.
The last condition is frequently confusing; it is because the vanishing of the (n − 1)-st homotopy group of the homotopy fiber Ff corresponds to a surjection on the nth homotopy groups, in the exact sequence:
If the group on the right vanishes, then the map on the left is a surjection.
- A connected map (0-connected map) is one that is onto path components (0th homotopy group); this corresponds to the homotopy fiber being non-empty.
- A simply connected map (1-connected map) is one that is an isomorphism on path components (0th homotopy group) and onto the fundamental group (1st homotopy group).
n-connectivity for spaces can in turn be defined in terms of n-connectivity of maps: a space X with basepoint x0 is an n-connected space if and only if the inclusion of the basepoint is an n-connected map. The single point set is contractible, so all its homotopy groups vanish, and thus "isomorphism below n and onto at n" corresponds to the first n homotopy groups of X vanishing.
This is instructive for a subset: an n-connected inclusion is one such that, up to dimension n−1, homotopies in the larger space X can be homotoped into homotopies in the subset A.
For example, for an inclusion map to be 1-connected, it must be:
- one-to-one on and
One-to-one on means that if there is a path connecting two points by passing through X, there is a path in A connecting them, while onto means that in fact a path in X is homotopic to a path in A.
In other words, a function which is an isomorphism on only implies that any element of that are homotopic in X are abstractly homotopic in A – the homotopy in A may be unrelated to the homotopy in X – while being n-connected (so also onto ) means that (up to dimension n−1) homotopies in X can be pushed into homotopies in A.
This gives a more concrete explanation for the utility of the definition of n-connectedness: for example, a space such that the inclusion of the k-skeleton in n-connected (for n>k) – such as the inclusion of a point in the n-sphere – means that any cells in dimension between k and n are not affecting the homotopy type from the point of view of low dimensions.
In geometric topology, cases when the inclusion of a geometrically-defined space, such as the space of immersions into a more general topological space, such as the space of all continuous maps between two associated spaces are n-connected are said to satisfy a homotopy principle or "h-principle". There are a number of powerful general techniques for proving h-principles.