At the beginning I should warn everybody reading this post: I don't know much about algebraic geometry so specialists in this subject may see my question as ignorant.

As far I understood one on the main themes in algebraic geometry is to pursue as far as it is possible the duality between geometric objects and algebras: most basic result is the Hilbert Nullstellensatz but the theory goes much further-to the definition of general *schemes* due to Grothendieck. The notion of *space* has evolved through the history of mathematics but as far as some topological space was around, the minimal requirement (at least for me) was that the space should be Hausdorff. This is quite natural due to the following characterisation: each net has at most one limit. Moreover, when one is interested in compact or locally compact spaces, the assumpion of being Hausdorff automatically implies better behaviour (normality or complete regularity resp.).

Finally, there is the theory (which is close to my heart) of $C^*$-algebras: in this theory a fundamental result is the Gelfand-Najmark theorem which establishes the duality between compact Hausdorff spaces and commutative unital $C^*$-algebras. This is another *algebra-geometry* duality and allows one to think of the theory of general $C^*$-algebras as *noncommutative topology*: but there are plenty of situations when one has a "pathological" topological space (with some non Hausdorff topology) which is hard to deal with. Then one switches to the realm of algebras and tries to say something about this space using the associated algebra: in other words one doesn't stick to a geometric picture.

It seems that algebraic geometry goes the other way around and works very often with topological spaces which are non-Hausdorff. So my (rather vague) question is the following:

**Question.** What is the *geometric* meaning and the intuition behind non-Hausdorff spaces in the realm of algebraic geometry? How to interpret such non Hausdorff topologies in this algebra-geometric context?

Let me give one example, which may clarify about what sort of things I'm asking: when one forms a quotient space one glues some points of the space to the another and in such a way one obtains a new set of points. In particular one can take some subset $A \subset X$ which is not closed and collapse it to the one point: then $X/A$ would be non Hausdorff and the special point in the quotient will be $\pi(a)$ where $a \in A$ is arbitrary and $\pi$ denotes the natural projection. My intuition behind this example is the following: the point $\pi(a) \in X/A$ was obtained from the richer set of data which was the set $A$ and the fact that $A$ was not closed. A more dramatic example would be $X/G$ where for each $g \in G$ its orbit is dense in $X$: then my intuition behind this example is the fact that the points in $X/G$ have some extra internal structure. So the operation of taking quotients very often gives a non-Hausdorff topology.

whythese spaces are non-Hausdorff? $\endgroup$ – Yemon Choi Jan 4 '15 at 20:47spaceswhich are not Hausdorff. As I explained before: in noncommutative topology when some space is non Hausdorff one rather try to investigate it by associating some algebra to it and then investigate this algebra. This is the first point. Second is that due to the existence of convergent nets with more than one limit my natural reaction to non Hausdorff spaces is to dismiss them as being pathological. By the way: maybe I should add "soft-question" tag to this question? $\endgroup$ – truebaran Jan 4 '15 at 21:00