Low Dimensional Topology

September 12, 2012

Subadditivity of complexities under gluing

Filed under: 3-manifolds,Knot theory — Jesse Johnson @ 1:17 pm

A recent talk in our topology seminar by Trent Schirmer (who just joined OSU as a postdoc this year) got me thinking about three closely related (almost equivalent) problems in three-dimensional topology. Trent spoke about the following problem in knot theory: Given two knots in S^3 with tunnel numbers t_1 and t_2, what would you expect the tunnel number of their connect sum to be? Recall that the tunnel number of a knot is the Heegaard genus of the knot complement minus one. With a little work, one can show that the tunnel number of the connect sum is at most t_1 + t_2 + 1. However, there are also examples where it is much lower and Trent has constructed links where the connect sum has tunnel number around \frac{4}{7}(t_1 + t_2) [1]. This is fairly interesting on its own, but it turns out there are (at least) two other situations with similar phenomenon that appear to have the same underlying reasons.

The second situation is as follows: Given two three-manifolds, each with a single torus boundary component, with Heegaard genera g_1 and g_2, what is the Heegaard genus of the manifold that results from gluing the manifolds along the tori by a given homeomorphism? In this case, amalgamating the Heegaard splittings gives you a Heegaard splitting of genus g_1 + g_2 - 1, and for “most” gluing maps, this is the Heegaard genus of the resulting manifold [2]. However, on the other end of the spectrum, Schultens-Weidmann have constructed examples where g_1 = g_2 and for a specific gluing map, the Heegaard genus of the resulting manifold is g_1 = g_2 = \frac{1}{2}(g_1 + g_2) [3]. Note that the conjectured minimum answer for the tunnel number problem is \max\{t_1, t_2\}, and I think a reasonable conjecture for the torus gluing problem would be \max\{g_1, g_2\}, which is realized by the Schultens-Weidmann construction. (By the way, this is a beautifully simple construction but unfortunately I think describing it here would make this post too long (if it isn’t already.)) The best known lower bounds for both problems are much lower: roughly \frac{2}{5} (t_1 + t_2) for connect sums by Scharlemann-Schultens [5] and \frac{1}{5} (g_1 + g_2) for torus gluing, which follows from work of Ryan Derby-Talbot [6].

So, what’s the connection between these two problems? In the second we’re gluing two manifolds together along a pair of tori. In the first, we’re taking a connect sum, but if you think of a knot connect sum as a three-manifold operation, you’re really gluing along a pair of annuli in the boundaries of the knot complements. Tori and annuli behave very similarly, so it’s not surprising that we would find a similar phenomenon in these two settings. But then there’s the third setting that I mentioned.

A generalized Conway product of two knots/links is the following construction: Given knots or links K_1 and K_2 in separate copies of the three-sphere, choose an arc with endpoints in each knot (and interior disjoint from each knot). A regular neighborhood of each arc is a ball and each knot intersects this ball in two unknotted arcs. Remove each of these balls from the appropriate copy of S^3 and glue the complements together along the resulting sphere boundary components so that the four endpoints of each knot match up. (Note that there is a choice of a braid along the gluing sphere.) The result is a knot or link in S^3, which is called a generalized Conway product.

Given knots with bridge numbers b_1 and b_2, the bridge number of the connect sum will always be b_1 + b_2 - 1. However, the bridge number of a generalized Conway product can be much lower. In particular, Ryan Blair has constructed examples where the bridge number of the generalized Conway product of two knots with bridge numbers b_1 = b_2 is b_1 [5]. Ryan also proved that the bridge number of the generalized Conway product cannot be lower than \min \{b_1, b_2\}, though \max \{b_1, b_2\} is probably a reasonable conjecture. So again, we have very similar subadditivity.

But how is this related to gluing along a torus or annulus? The answer seems to be that when you compare bridge surfaces to Heegaard surfaces (which also covers tunnel number), you should work in the double branched cover. As I’ve written about before, a bridge surface lifts to a Heegaard surface in the double branched cover of S^3 branched over the knot. The genus of this Heegaard surface is the bridge number minus one. The four punctured sphere defined by the generalized Conway product lifts to a torus. So the subadditivity in this setting appears to be caused by the same type of phenomenon as in the other two, if you know how to look at it.

What I find really interesting is that each of the three settings suggests different techniques, and these different techniques have resulted in very different examples and very different lower bounds. But somehow, the underlying mechanics of the situation, which these different techniques try to exploit, seem to be very much the same.


  1. Jesse: Morimoto and Schultens [a] showed that for small knots in S^3, the tunnel number of the connected sum is at least t_1 + t_2. From your blog’s POV it’s the analogue of [2]. Kobayashi and I generalized it [b].

    [a] Morimoto, Kanji; Schultens, Jennifer: Tunnel numbers of small knots do not go down under connected sum. Proc. Amer. Math. Soc. 128 (2000), no. 1, 269-278.

    [b] Kobayashi, Tsuyoshi; Rieck, Yo’av: Heegaard genus of the connected sum of m-small knots. Comm. Anal. Geom. 14 (2006), no. 5, 1037-1077.

    Comment by Yo'av Rieck — September 26, 2012 @ 9:44 am | Reply

  2. Small knots are prime. So what about prime knots? http://repositories.lib.utexas.edu/handle/2152/ETD-UT-2011-12-4617

    Comment by anon — January 12, 2013 @ 12:19 pm | Reply

    • Prime knots may or may not be small. I’m not sure what degeneration ratio Noquiera gets in the dissertation you link to, but by Morimoto-Schultens and Kobayashi-Rieck these examples must not be meridionally small.

      Comment by Jesse Johnson — February 10, 2013 @ 4:15 pm | Reply

RSS feed for comments on this post. TrackBack URI

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

Blog at WordPress.com.

%d bloggers like this: