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> If $S$ is the surface illustrated below, do the Dehn twists about the red curves generate the mapping class group $\operatorname{MCG}(S,\partial S)$?
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<http://math.stanford.edu/~pardon/img/dehntwists.png>



Do the following set of Dehn twists generate the mapping class group?
No. If they did, then they would still generate the mapping class group of the closed surface that results from gluing a disc to the boundary component. However, in that surface they all commute with the hyperelliptic involution, which is not central for $g$ at least $3$.


In fact, Humphries proved that you need at least $2g+1$ Dehn twists to generate the mapping class group. This is contained somewhere in Farb-Margalit's primer on mapping class groups.


In the closed surface, these Dehn twists actually generate the centralizer of the hyperelliptic involution, which is known as the hyperelliptic mapping class group. This is a theorem of Birman and Hilden, which can also be found somewhere in Farb-Margalit.


