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(1) Given a fundamental group representation of a hyperbolic surface, i.e. $<a_j,b_j|\prod[a_j,b_j]=1>$, and given an element in this group, can we determine whether this element can be represented by a simple closed curve?

(2) More specifically, if we consider only a commutative element, whose abelizaions is trivial in the homology group, is there some method to determine whether this element can be represent by a simple closed separating curve on the surface?

For example, $a_1a_2a_1^{-1}a_2^{-1}$ cannot be represented by a simple closed separating curve. I guess the geometric interpretation of a simple closed separating curve is that it is the boundary of the neibourhood of a group of chained curves. Here a group of chained curves means a set of curves $\{c_1, \dots ,c_{2h}\}$ satisfying the following conditions: the geometric intersection numbers are $i(c_j,c_{j+1})=1$ and $i(c_j,c_{k})=0$ for $k-j>1$. But even this criterion is true, the choice of $c_j$'s in the fundamental group are diverse. From an element in the fundamental group, we may not easily see that whether it is the boundary of a group of chained curves. Are there some easy way or algorithm to determine it?

The previous discussion is for question (2). What if we consider the elements in question (1)? I learn from Farb and Margalit's book that there is a neccesary condition on the homology group that the abelization element of a simple closed curve in the homology group must be primitive. What about the representation in the fundamental group?

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4 Answers 4

up vote 8 down vote accepted

There is no simple necessary and sufficient condition for whether an element of the fundamental group can be realized by a simple closed curve. However, there are a variety of algorithms known. I believe that the first such algorithm is given in

Reinhart, Bruce L. Algorithms for Jordan curves on compact surfaces. Ann. of Math. (2) 75 1962 209–222.

But probably the most used one is the one in

Birman, Joan S.; Series, Caroline, An algorithm for simple curves on surfaces. J. London Math. Soc. (2) 29 (1984), no. 2, 331–342.

It uses hyperbolic geometry.

I should remark that both of these papers concern unbased curves; however, if $\Sigma_g$ is a closed genus $g$ surface, then an element $\gamma \in \pi_1(\Sigma_g,\ast)$ can be realized by a based simple closed curve if and only if it is freely homotopic to a simple closed curve. Indeed, if $\gamma$ is freely homotopic to a simple closed curve, then there is some $x \in \pi_1(\Sigma_g,\ast)$ such that $x \gamma x^{-1}$ can be realized by a based simple closed curve. But it follows from the Dehn-Nielsen-Baer theorem that inner automorphisms of $\pi_1(\Sigma_g,\ast)$ can be realized by based homeomorphisms of the surface, so thus $\gamma$ can also be realized by a based simple closed curve.

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(it looks like HJRW posted some of this info while I was typing, but I'll leave this up since it does contain some other stuff). –  Andy Putman Sep 19 '13 at 15:04
    
Snap!${}{}{}{}$ –  HJRW Sep 19 '13 at 15:34
    
@HJRW : Wait, was my comment inadvertently mean? I was just trying to acknowledge your priority. Certainly no offense was intended. –  Andy Putman Sep 19 '13 at 15:46
    
I meant 'snap' in the English sense: en.wikipedia.org/wiki/Snap_%28card_game%29#Snap Sorry, I seem to have caused confusion. –  HJRW Sep 19 '13 at 15:49
2  
@Andy - I think that "snap" in English translates to "jinx" in American. separatedbyacommonlanguage.blogspot.co.uk/2006/10/… –  Sam Nead Jan 2 at 21:43

Various algorithms for determining whether a given conjugacy class contains a simple representative are given in the following papers.

Reinhart, Bruce L., 'Algorithms for Jordan curves on compact surfaces', Ann. of Math. (2) 75 1962 209–222.

Zieschang, Heiner, 'Algorithmen für einfache Kurven auf Flächen', (German) Math. Scand. 17 1965 17–40.

Birman, Joan S; Series, Caroline, 'An algorithm for simple curves on surfaces', J. London Math. Soc. (2) 29 (1984), no. 2, 331–342.

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A recent paper by Patricia Cahn, A Generalization of the Turaev Cobracket and the Minimal Self-Intersection Number of a Curve on a Surface gives a strengthening of Turaev's bracket. She shows that a related invariant of a free homotopy class is 0 if and only if the class is a power of a simple loop. This answers your question, modulo determining if your class is a proper power of some other element; I have no idea how difficult this latter is, or indeed how calculable her invariant is. There are some worked examples in the paper.

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A necessary condition is that Turaev's cobracket is zero. See the paper of Moira Chas on combinatorial Lie bialgebras. There is also a later paper by Chas and Krongold, which promises an answer to your question, but I have not read it.

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