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''of two continuous mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479201.png" />''

A formalization of the intuitive idea of deformability of one mapping into another. More exactly, two mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479202.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479203.png" /> are called homotopic (denoted by <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479204.png" />) if there exists a family of continuous mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479205.png" />, continuously depending on a parameter <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479206.png" />, such that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479207.png" />, <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479208.png" /> (the choice of the segment <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h0479209.png" /> is here only for the sake of technical convenience; it is clear that any other segment of the real axis could have been taken just as well). This family (which is known as a homotopy connecting <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792010.png" /> with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792011.png" />) is a path in the space <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792012.png" /> of all continuous mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792013.png" /> connecting the point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792014.png" /> with the point <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792015.png" />. Thus, a homotopy of mappings is a specialization to the space of mappings of the general concept of  "being connected by a continuous path" . Thus, in particular, the homotopy relation is an equivalence relation, whose equivalence classes (homotopy classes) are the path-connected components of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792016.png" />. In order to give an exact meaning to the above-said one must specify the meaning of the expression  "the mappings $f_t$ continuously depend on t" . The most natural procedure is to introduce a topology (or at least a pseudo-topology, cf. also [[Topological structure (topology)|Topological structure (topology)]]) in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792017.png" />. However, the traditional procedure is different — it is assumed, by definition, that the <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792018.png" /> continuously depend on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792019.png" /> if the function <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792020.png" /> is continuous with respect to the totality of variables, i.e. if the mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792021.png" /> defined by the formula <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792022.png" /> is continuous (this mapping is in fact often referred to as a homotopy connecting <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792023.png" /> with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792024.png" />).

This description of a homotopy is sometimes qualified as free, in distinction from  "relative homotopyrelative"  or  "bound homotopybound"  homotopies, which arise upon fixing a class <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792025.png" /> of continuous mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792026.png" />, by imposing the requirement <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792027.png" /> for any <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792028.png" />. Thus, given a subspace <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792029.png" /> it is possible to consider relative homotopies on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792030.png" />, distinguished by the fact that <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792031.png" /> on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792032.png" /> for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792033.png" />. One says in this case that the mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792034.png" /> is homotopic with the mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792035.png" /> relative to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792036.png" />; this is written as <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792037.png" /> <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792038.png" />.

Another type of  "relative"  homotopies is generated by selecting subspaces <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792039.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792040.png" /> in <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792041.png" /> and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792042.png" /> and by considering only mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792043.png" /> that satisfy the condition <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792044.png" />. Such mappings are called mappings of the pair <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792046.png" /> into the pair <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792047.png" /> (written as <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792048.png" />), while the corresponding homotopies (i.e. the homotopies for which <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792049.png" /> for all <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792050.png" />) are known as homotopies of pair mappings. Triplets <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792051.png" /> (with the condition <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792052.png" /> or without it), quadruplets, etc., rather than pairs, can also be considered. Thus, one can consider homotopy of pair mappings relative to a third subspace, etc. Essentially different types of  "relative"  homotopies may also be considered.

The task of establishing homotopy ( "relative"  or not) of two given mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792053.png" /> is equivalent to the task of extending to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792054.png" /> a continuous mapping into <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792055.png" /> given on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792056.png" /> (and, in a homotopy problem <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792057.png" />, on <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792058.png" />). In this sense, the problem of homotopy is a special case of the problem of extension. In a wide class of individual cases, however (viz. for the so-called cofibrations, cf. [[Cofibration|Cofibration]]), the possibility of extending to <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792059.png" /> a continuous mapping <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792060.png" /> given on a subspace <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792061.png" /> depends only on its homotopy class. This close connection between the problem of homotopy and the problem of extension is the reason why they are considered together under the heading of so-called homotopy theory. See [[Homotopy type|Homotopy type]].



====Comments====
The context in which the term  "free homotopy"  is used differs slightly in the West; one uses it in the context of pointed spaces (a pointed space is a pair <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792062.png" /> with <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792063.png" /> a space and <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792064.png" /> a point of <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792065.png" />). Here then a  "free"  homotopy is a homotopy between arbitrary mappings, whereas for an  "ordinary"  homotopy one considers mappings <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792066.png" /> satisfying <img align="absmiddle" border="0" src="https://www.encyclopediaofmath.org/legacyimages/h/h047/h047920/h04792067.png" /> and the mappings in the homotopies are to satisfy this condition too.

====References====
<table><TR><TD valign="top">[a1]</TD> <TD valign="top">  G.W. Whitehead,   "Elements of homotopy theory" , Springer  (1978)  pp. 228</TD></TR></table>