# Core-compact space

Let $X$ be a topological space with $\mathfrak{O}_X$ the collection of open sets. If $U, V$ are open, we say that $U$ is compact in $V$ if every open cover of $V$ has a finite subset that covers $U$. The space $X$ is core compact if for any $x \in X$ and open neighbourhood $N$ of $x$, there is an open set $V$ such that $N$ is compact in $V$.

A space is core compact if and only if $\mathfrak{O}_X$ is a continuous lattice. A locally compact space is core compact, and a sober space (and hence in particular a Hausdorff space) is core compact if and only if it is locally compact.

A space is core compact if and only if the product of the identity with a quotient map is quotient.

# Developable space

A development in a topological space $X$ is a sequence of open covers $G_n$ such that for all points $x \in X$ the stars $$\mathrm{St}(x,G_n) = \cup \{ U \in G_n : x \in U \}$$ form a local base for $x$. A developable space is a space with a development. A metric space is a developable space: the sequence of collections of open balls of radius $1/n$ forming a development. A Moore space is a regular space with a development. A collection-wise normal Moore space is metrizable.

A regular development has the further property that if $U,V \in G_{n+1}$ with $U \cap V \neq \emptyset$, then there is $W \in G_n$ with $U \cup V \subset W$. Alexandroff and Urysohn proved that a space is metrizable if and only if it has a regular development.

# Approach space

A generalisation of the concept of metric space, formalising the notion of the distance from a point to a set. An approach space is a set $X$ together with a function $d$ on $X \times \mathcal{P}X$, where $\mathcal{P}X$ is the power set of $X$, talking values in the extended positive reals $[0,\infty]$, and satisfying $$d(x,\{x\}) = 0 \ ;$$ $$d(x,\emptyset) = \infty \ ;$$ $$d(x,A\cup B) = \min(d(x,A),d(x,B)) \ ;$$ $$d(x,A) \le d(x,A^u) + u \ ;$$ where for $u \in [0,\infty]$, we write $A^u = \{x \in X : d(x,A) \le u \}$.

A metric space $(X,\delta)$ has an approach structure via $$d(x,A) = \inf\{ \delta(x,a) : a \in A \} \ .$$ and a topological space $X,{}^c$, where ${}^c$ is the Kuratowksi closure operator, via $$d(x,A) = \begin{cases} 0 & \ \text{if} x \in A^c \\ \infty & \ \text{otherwise} \end{cases} \ .$$

#### References

• Hofmann, Dirk (ed.); Seal, Gavin J. (ed.); Tholen, Walter (ed.) "Monoidal topology. A categorical approach to order, metric, and topology" Cambridge University Press (2014) ISBN 978-1-107-06394-5 Zbl 1297.18001

# Ample field

A field which is existentially closed in its field of formal power series. Examples include pseudo algebraically closed fields, real closed fields and Henselian fields.

A field $K$ is ample if and only if every absolutely irreducible curve over $K$ with a simple $K$-point has infinitely many $K$-points.

# Binary tetrahedral group

The exceptional group $G_4$ or $\langle 3,3,2 \rangle$, abstractly presented as: $$\langle R,S \ |\ R^3=S^3=(RS)^2 \rangle \ .$$ It is finite of order 24. It has the alternating group $A_4$ as quotient by the centre and the quaternion group of order 8 as a quotient.

This group may be realised as the group of invertible Hurwitz numbers: $$\pm 1\,,\ \pm i\,,\ \pm j\,,\ \pm k\,,\ \frac{\pm1\pm i\pm j\pm k}{2} \ .$$

The group has an action on the three-sphere with tetrahedral space as quotient.

#### References

 [a1] H.S.M. Coxeter, "Regular complex polytopes" , Cambridge Univ. Press (1991) pp. 76 ISBN 0-521-20125-X Zbl 0732.51002

# Binary icosahedral group

The group $\langle 5,3,2 \rangle$ abstractly presented as: $$\langle A,B \ |\ A^5=B^3=(AB)^2 \rangle \ .$$ It is finite of order 120.

The group has an action on the three-sphere with dodecahedral space as quotient.

#### References

 [a1] H.S.M. Coxeter, "Regular complex polytopes" , Cambridge Univ. Press (1991) pp. 77 ISBN 0-521-20125-X Zbl 0732.51002

# Binary octahedral group

The group $\langle 4,3,2 \rangle$ abstractly presented as: $$\langle A,B \ |\ A^4=B^3=(AB)^2 \rangle \ .$$ It is finite of order 48. It has the binary tetrahedral group $G_4 = \langle 3,3,2 \rangle$ as a subgroup of index 2.

The group has an action on the three-sphere with octahedral space as quotient.

#### References

 [a1] H.S.M. Coxeter, "Regular complex polytopes" , Cambridge Univ. Press (1991) pp. 77 ISBN 0-521-20125-X Zbl 0732.51002

# Dodecahedral space

The result of identifying opposite faces of a dodecahedron by a right-handed turn of angle $\pi/5$. It is the quotient of the three-sphere by the binary icosahedral group.

Dodecahedral space is a homology sphere (Poincaré sphere).

# Étale algebra

A commutative algebra $A$ finite-dimensional over a field $K$ for which the bilinear form induced by the trace $$\langle x,y \rangle = \mathrm{tr}_{A/K} (x\cdot y)$$ is non-singular. Equivalently, an algebra which is isomorphic to a product of field $A \sim K_1 \times \cdots \times K_r$ with each $K_i$ an extension of $K$.

Since $\langle xy,z \rangle = \mathrm{tr}(xyz) = \langle x,yz \rangle$, an étale algebra is a Frobenius algebra over $K$.

# Unit quaternion

A quaternion with norm 1, that is, $x_i + yj + zk + t$ with $x^2+y^2+z^2+t^2 = 1$.

The real unit quaternions form a group isomorphic to the special unitary group $\mathrm{SU}_2$ over the complex numbers, and to the spin group $\mathrm{Sp}_3$. They double cover the rotation group $\mathrm{SO}_3$ with kernel $\pm 1$.

The finite subgroups of the unit quaternions are given by group presentations $$A^p = B^q = (AB)^2$$ with $1/p + 1/q > 1/2$, denoted $\langle p,q,2 \rangle$. They are

#### References

 [a1] H.S.M. Coxeter, "Regular complex polytopes" , Cambridge Univ. Press (1991) Zbl 0732.51002

# Dicyclic group

A finite group of order $4n$, obtained as the extensions of the cyclic group of order $2$ by a cyclic group of order $2n$. It has the presentation $\langle n,2,2 \rangle$ and group presentation $$A^n = B^2 = (AB)^2 \ .$$ It may be realised as a subgroup of the unit quaternions.

The dicyclic group $n=2$ is the quaternion group of order $8$.

#### References

 [a1] H.S.M. Coxeter, "Regular complex polytopes" , Cambridge Univ. Press (1991) Zbl 0732.51002
How to Cite This Entry:
Richard Pinch/sandbox-9. Encyclopedia of Mathematics. URL: http://www.encyclopediaofmath.org/index.php?title=Richard_Pinch/sandbox-9&oldid=42431