This week’s SUMS talk is by postgrad Chris Ryba. Pizza and drinks will as always be available afterwards. Abstract: Some differential equations are so messy that they can’t be written down in a nice form, leaving little hope that their solutions are manageable. Fortunately, using the chain rule in clever ways, it turns out that we can find approximate solutions by counting trees. It’s even possible to reverse the process and use differential equations to count particular types of trees.