Base of a topology I am confused with the concept of topology base. Which are the properties a base has to have?
Having the next two examples for $X=\{a,b,c\}$:
1) $(X,\mathcal{T})$ is a topological space where $\mathcal{T}=\{\emptyset,X,\{a\},\{b\},\{a,b\}\}$. Which is the general procedure to follow in order to get a base for $(X,\mathcal{T})$? Can $\mathcal{B}=\{\{a\},\{b\}\}$ be a base?
2) Having just $X$ and no topology $\mathcal{T}$ defined for $X$, is $\mathcal{A}=\{X,\{a\},\{c\}\}$ a base? What is the topology that it generates? 
Thank you very much.
 A: Let $X$ be a non-empty set. A collection $\mathscr{B}$ of subsets of $X$ is a base for some topology on $X$ if it satisfies two conditions:


*

*$\mathscr{B}$ covers $X$. That is, every point of $X$ belongs to at least one member of $\mathscr{B}$.  

*If $B_1,B_2\in\mathscr{B}$ and $x\in B_1\cap B_2$, then there is a $B_3\in\mathscr{B}$ such that $x\in B_3\subseteq B_1\cap B_2$.


These two conditions are exactly what is needed to ensure that
$$\mathscr{T}=\Big\{\bigcup\mathscr{A}:\mathscr{A}\subseteq\mathscr{B}\Big\}$$
is a topology on $X$. In words, the collection of all unions of members of $\mathscr{B}$ is a topology on $X$, the topology generated by the base.
Note that a topology may have many different bases. The topology $\big\{\varnothing,\{a\},\{b\},\{a,b\}\big\}$ on the set $\{a,b\}$ has the following bases:


*

*$\big\{\varnothing,\{a\},\{b\},\{a,b\}\big\}$  

*$\big\{\{a\},\{b\},\{a,b\}\big\}$  

*$\big\{\varnothing,\{a\},\{b\}\big\}$

*$\big\{\{a\},\{b\}\big\}$


Conditions (1) and (2) above are the easiest way to characterize the families of sets that are bases for some topology on $X$. If you already have a topology $\mathscr{T}$ on $X$, you can say simply that a subset $\mathscr{B}$ of $\mathscr{T}$ is a base for $\mathscr{T}$ if and only if every member of $\mathscr{T}$ (i.e., every open set in the space $\langle X,\mathscr{T}\rangle$) is a union of members of $\mathscr{B}$.

Yes, $\big\{\varnothing,\{a\},\{c\},X\big\}$ is a base for a topology on $X$: it satisfies the two conditions given at the beginning of this answer. The topology that it generates is
$$\big\{\varnothing,\{a\},\{c\},\{a,c\},\{a,b,c\}\big\}\;.$$
A: A base is a collection $B$ of sets such that every set in the topology can be written as a union of sets in $B$.
1) Can you write every set in $T$ as a union of $\{a\}$ and/or $\{b\}$?
2) Produce all the unions then you'll see what the topology is.
A: Let $(X,\tau)$ be a topological space and $\mathcal{B}\subseteq 2^X$.
$$\mathcal{B} \text{ is a base for topology } \tau \text { on } X$$
$$:\Leftrightarrow$$
$$1) \mbox{ } \mathcal{B}\subseteq \tau$$
$$2) \mbox{ } (\forall A\in\tau)(\exists\mathcal{A}\subseteq\mathcal{B})(A=\cup\mathcal{A})$$
