## Welcome to Yongheng Zhang's Homepage!

### Interests

Algebraic topology. Homotopy theory. And applications. I think about configuration spaces (and their variants) of points in spaces and using them to probe questions in algebra, geometry and topology.

### Publication

Kaufmann, Ralph M.; Zhang, Yongheng Permutohedral structures on E2-operads. Forum Math. 29 (2017), no. 6, 1371-1411.

...

### Current Teaching (Fall  2020)

• MATH 211  Multivariable Calculus

• via ZOOM

### Office Location

• Seeley Mudd 510; teaching from home

### Teaching at Purdue

• Geometry for Elementary Teachers
• Topology for High School Students (notes by Hannah Burnau)
• Calculus I, II, III
• Linear Algebra
• ODE and basic PDE

### Tokyo 2020 Emblems

and how they (Olympic, Paralympic) look like if drawn by straightedge and compass

## Moon's orbit around star

Q: A moon revolves around a planet as the planet revolves around a star. Assume that both orbits are circular and they are on the same plane, then which of the following curves could be the orbit of the moon around the star?

A: All are possible. A student would be able to give a precise analytical explanation within the first three weeks of taking multivariable calculus (using derivative, cross product and imagination), but if you'd like to see a quick proof without using algebra/calculus, click on the picture below. ## When we integrate A along the boundary of B ...

We are also integrating the "boundary" (the differential) of A over B. This is the principle behind the generalized Stokes' Theorem, which put the Fundamental Theorem of Calculus, the Fundamental Theorem of Line Integrals, Green's Theorem, Stokes' Theorem and Gauss's (Divergence) Theorem all under its aegis. Curiously, some mechanical devices also employ this principle. In the following picture, if you move the wheel at the end of the long arm along the green curve once while keeping the cart on the black track, the number you would get after multiplying the total wheel mileage with the length of the arm is the area of the region bounded by the green curve! Indeed, how it works can be explained via Green's Theorem. You can learn it and much more from Prof. Tanya Leise's article linked below.

Leise, Tanya As the planimeter's wheel turns: planimeter proofs for calculus class. College Math. J. 38 (2007), no.1, 24-31. ## __________________________________________________________

The mechanical device shown in the above picture is one of the simplest types of planimeter, yet it can measure area of a variety of regions, as long as the following three conditions hold.

• We can fit the region into an infinitely long strip with width double the length of the measuring arm.
• The boundary of the region is piecewise smooth.
• The arm can only rotate in the forward 180 degrees range so that it never passes its shoulder. The reason is to get the abstract vector field associated to the measuring wheel always well-defined. One could also explain it using topological winding number, as already hinted in Tanya Leise's article.

The following animation shows how the device moves in order to measure the area of a disk with largest possible radius. Notice that the cart has to move during the second half of the trip in order to keep the third promise above.

In practice, one has to move the location of the measuring wheel or to change this subsystem to other recording mechanism, but the above version is simplest to understand if you want to lay out all the details, which a student would be able to  do within the last three weeks of taking multivaraible calculus. ## How to reversely park a truck-trailer system?

I thought about this problem shortly after I moved from West Lafayette to Amherst around July 3 (?) of 2015. Click on the picture below to see a method, which is the result of solving a system of differential equations with constrained inputs, sprinkled with a little analysis, some elementary geometry and a bit of numerial computation. Dimensions are based on real data from U-Haul's website. 