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**Notes on AKT-140117:**
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Euler-Lagrange problems, Gaussian integration, volumes of spheres.

# | Week of... | Notes and Links |
---|---|---|

1 | Jan 6 | About This Class (PDF). Monday: Course introduction, knots and Reidemeister moves, knot colourings. Tricolourability without Diagrams Wednesday: The Gauss linking number combinatorially and as an integral. Friday: The Schroedinger equation and path integrals. Friday Introduction (the quantum pendulum) |

2 | Jan 13 | Homework Assignment 1. Monday: The Kauffman bracket and the Jones polynomial. Wednesday: Self-linking using swaddling. Friday: Euler-Lagrange problems, Gaussian integration, volumes of spheres. |

3 | Jan 20 | Homework Assignment 2. Monday: The definition of finite-type and some examples. Wednesday: The self-linking number and framings. Friday: Integrating a polynomial times a Gaussian. Class Photo. |

4 | Jan 27 | Homework Assignment 3. Monday: Chord diagrams and weight systems. Wednesday: Swaddling maps and framings, general configuration space integrals. Friday: Some analysis of . |

5 | Feb 3 | Homework Assignment 4. Monday: 4T, the Fundamental Theorem and universal finite type invariants. The Fulton-MacPherson Compactification (PDF). Wednesday: The Fulton-MacPherson Compactification, Part I. Friday: More on pushforwards, , and . |

6 | Feb 10 | Homework Assignment 5. Monday: The bracket-rise theorem and the invariance principle. Wednesday: The Fulton-MacPherson Compactification, Part II. Friday: Gauge fixing, the beginning of Feynman diagrams. |

R | Feb 17 | Reading Week. |

7 | Feb 24 | Monday: A review of Lie algebras. Wednesday: Graph cohomology and . Friday: More on Feynman diagrams, beginning of gauge theory. From Gaussian Integration to Feynman Diagrams (PDF). |

8 | Mar 3 | Homework Assignment 6 (PDF) Monday: Lie algebraic weight systems. Wednesday: Graph cohomology and the construction of . Graph Cohomology and Configuration Space Integrals (PDF) Friday: Gauge invariance, Chern-Simons, holonomies. Mar 9 is the last day to drop this class. |

9 | Mar 10 | Homework Assignment 7 (PDF) Monday: The weight system. Wednesday: The universal property, hidden faces. Friday: Insolubility of the quintic, naive expectations for CS perturbation theory. |

10 | Mar 17 | Homework Assignment 8 (PDF) Monday: and PBW. Wednesday: The anomaly. Friday: Faddeev-Popov, part I. Gaussian Integration, Determinants, Feynman Diagrams (PDF). |

11 | Mar 24 | Homework Assignment 9 (PDF) Monday: is a bi-algebra. Wednesday: Understanding and fixing the anomaly. Friday: class cancelled. |

12 | Mar 31 | Monday, Wednesday: class cancelled. Friday: A Monday class: back to expansions. |

E | Apr 7 | Monday: A Friday class on what we mostly didn't have time to do. |

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$\frac{d}{d\epsilon}f(\epsilon)\mid_{\epsilon = 0} = \frac{mg}{s}\int_{-L}^{L}y_q\sqrt{1+\dot{y}_q^2}+y_c(1+\dot{y}_c^2)^{\frac{-1}{2}(\dot{y}_c\dot{y}_q)}dx = 0$

Thus, $0 = \frac{mg}{s}(I_1 + I_2)$, where $I_1 = \int_{-L}^L y_q\sqrt{1+\dot{y}_c^2}dx$ and $I_2 = \int_{-L}^L y_c\dot{y}_c\dot{y}_q(1+\dot{y}_c^2)^{\frac{-1}{2}}dx$.

Integrating $I_2$ by parts with $u = y_c\dot{y}_c(1+\dot{y}_c^2)^{\frac{-1}{2}}$ and $dv = \dot{y}_q dx$, and applying boundary conditions $y_q(-L) = y_q(L) = 0$, we obtain

$I_2 = \int_{-L}^L y_q\big(y_c\dot{y}_c^2\ddot{y}_c(1+\dot{y}_c^2)^{\frac{-3}{2}} - (\dot{y}_c^2 + y_c\ddot{y}_c)(1+\dot{y}_c^2)^{\frac{-1}{2}})\big)dx$

From $I_1 + I_2 = 0$, factoring out the $y_q$ and the fundamental lemma of the calculus of variations, we obtain an ODE (replacing $y_c$ with $y$):

$0 = (1+\dot{y}^2)^\frac{-3}{2}((1+\dot{y}^2)^2 - (\dot{y}^2 + y\ddot{y})(1+\dot{y}^2) + y\dot{y}^2\ddot{y})$

Dividing through by $(1+\dot{y}^2)^\frac{-3}{2}$, expanding, and simplifying, we obtain our final ODE:

$1 + \dot{y}^2 - y\ddot{y} = 0$

A solution to this is $y(x) = \frac1{\lambda}cosh(\lambda x + c)$, where $\lambda$ and c are determined by physical constants and the boundary values of $y$ (at $x = -L$ and $x = L$). It turns out that this ODE is the same one you get when solving the soap bubble problem (HW 2, problem 3), since the Lagrangians of the two systems are the same up to constants.

Let . This now becomes a single variable minimum/maximum problem. We set , and solve for . First, simplifying , we compute

+ higher order terms).

Thus,

Integrating by parts with , this is equal to

The first term is equal to 0 by boundary conditions of , so we obtain the equality

, exactly as stated in the conclusion of Lemma 3.4. Solving this ODE with initial conditions gives the desired result. Explicitly, the solution of this ODE (with ) is

Plugging in and , we have

and , implying that , as claimed.

where are the initial time and final time, respectively. The integrand is known as the **Lagrangian** and is assumed to be time-independent for convenience. The idea here is to find the path that minimize the action . Now, we introduction the idea of variation, which can be viewed as an infinitesimal shift from the original path; however, it does not change the terminal points. Since the path we are interested is the path that minimizes the action, then the variation of the action should be 0 and that is

Then, by the integration by parts, we have that

since the boundary term does not vary so that . Thus, we arrive at the point where the classical particle must obey the path where the equation

This equation is known as the Euler-Lagrange equation.

Thus, we have

Then, the time may be described as

where is the infinitesimal arclength of the path. Then, let be the horizontal coordinate, we have Thus, the above equation would be

Now, let , we apply the Euler-Lagrange equation and obtain

If we rearrange the equation and integrate, we obtain the equation

where is some constant. Then, we rearrange the equation and obatin

Then, we can solve this equation with parameterization and obtain the final result