# Assignment -1: Favorite Formula

Part of your course grade is determined by participation, which can include both in-class participation as well as discussion here on the course webpage.  Therefore, your first assignment is to:

1. create an account (you must use your Andrew email address, so we can give you participation credit this semester!),
To make things interesting, your comment should include a description of your favorite mathematical formula typeset in $\LaTeX$.  If you don’t know how to use $\LaTeX$ this is a great opportunity to learn — a very basic introduction can be found here.  (And if you don’t have a favorite mathematical formula, this is a great time to pick one!)

(P.S. Anyone interested in hearing about some cool “favorite theorems” should check out this podcast.)

## 49 thoughts on “Assignment -1: Favorite Formula”

1. ecdeo says:

$e^{i\pi} + 1 = 0$

2. E says:

$$Fib(n) = \frac{\phi^n – (-\frac{1}{\phi})^n}{\sqrt{5}}$$

3. bce says:

$\sin^2(\theta) + \cos^2(\theta) = 1$

4. cheep says:

$$\sum_{i=1}^{\infty} \frac{1}{i^2} = \frac{\pi^2}{6}$$

5. imdabun says:

Change of Variables
$\int_{g(E)} f(y) dy = \int_E f(g(x)) |\text{det} J_g (x)| dx$

6. gwhitfie says:

$L_0(p, w_0) = L_e(p, w_0) + \int_{H^2} \! f_r(p, w_i \rightarrow w_0)L_i(p, w_i)cos(\theta) \, \mathrm{d}w_i$

7. Krishna says:

$$V – E + F = 2$$
i.e. a polyhedron has 2 fewer edges than vertices and faces. Or more generally
$$V – E + F = \chi$$
This defines the Euler characteristic, a topological invariant which can be broadly generalized in fascinating ways to prove seemingly unrelated theorems, such as “you can’t comb flat the hair on a coconut, but you could do the same for a torus.”

8. Alex says:

$$\lim_{n\rightarrow \infty} Pr(|X_n – \mu| > \epsilon) = 0$$

9. cbyh says:

Inspired by the Euler Characteristic mentioned above, here is another form:
$\mathcal{X}(V_{\centerdot})=\sum_{i}(-1)^{i}dim(V_i)=\sum_{i}(-1)^{i}dim(H_{i}(V_{\centerdot}))$ where $V_{\centerdot}$ is a complex of finite-dimensional vector spaces and $H_i$ is homology group.

10. ZackLee says:

$a^2 + b^2 = (a+b)^2$

1. Contracrostipunctual says:

It’s true in a ring of characteristic 2!

11. Sree says:

Triangle Inequality:
$||x+y|| \le ||x|| + ||y||$

12. praneeth says:

$\|A\|_F^2 = \|A(I-B^+B)\|_F^2 + \|AB^+B\|_F^2$

13. vinodkri says:

$$\iiint_E \textrm{div}(\vec{F})\textrm{d}V = \iint_{\partial E} \vec{F} \cdot \textrm{d}A$$

14. bmmiller says:

$$u(x) = \int_{\partial\Omega} \frac{dG(x,y)}{dn} u(y) – \frac{du(y)}{dn} G(x,y) dy$$

15. Max says:

$$(\lambda x.M[x])\to(\lambda y.M[y])$$

16. bamado says:

$\int_{- \infty}^{\infty} e^{-x^2} dx = \sqrt{\pi}$

17. cslamber says:

$$x \cdot y = \Vert x \Vert \Vert y \Vert \cos \theta$$
makes comp geo substantially more fun when you can do trig-related things staying in integers/with few operations.

18. borderwing says:

$\sum_{n=1}^{\infty}\frac{1}{n^2}=\frac{\pi^2}{6}$

19. Arp says:

Hardy-Weinberg equilibrium: $p^2 + 2pq + q^2 = 1$

20. Jim says:

Pythagorean Theorem:
For a right triangle,
$a^2+b^2=c^2$
where $a$ = side of right triangle,
$b$ = side of right triangle,
$c$ = hypotenuse

21. krisL says:

$x_{n+1} = rx_{n}(1 – x_{n})$

22. holly says:

$1 + 2 + … + n = \frac{n(n+1)}{2}$

23. Abigail Rafter says:

Euler’s Formula: $$e^{ix} = cos(x) + isin(x)$$

24. tmeringenti says:

$1 + \frac{1}{2} + \frac{1}{3} + \frac{1}{4} + \frac{1}{5} + … = \infty$

25. acdhia says:

$$\sum_{i=1}^{n} i = \frac{n(n+1)}{2}$$

26. sywang says:

Einstein’s field equation:
$$R_{\mu \nu} – \frac{1}{2}Rg_{\mu \nu} = \frac{8\pi G}{c^4}T_{\mu \nu}$$ . Metric rules!

27. sbala says:

$$d = \sqrt{(x_2 – x_1)^2 + (y_2 – y_1)^2)}$$

28. nmass says:

$$(x+y)^n = \sum\limits_{k=0}^n\binom{n}{k}x^ky^{n-k}$$

29. jabrams says:

Not sure it’s my favorite, but I don’t think anyone’s used it and I’m fond of the Gaussian integral:

$$\int_{-\infty}^\infty e^{-x^2}dx = \sqrt{\pi}$$

30. allai5 says:

Fourier Transform:
$X(f) = \int_{}{} x(t) \times e^{-i2 \pi ft} \,dt$

Not like I use it a lot (if at all) on a daily basis but give my lack of knowledge of complex numbers I find it pretty interesting that we can use imaginary numbers and such in practical applications.

31. Sherwin Jin says:

Mass-energy equivalence:
$E = mc^2$

32. wisepigeon says:

The number of edges in a plane triangulation (planar graph with the most possible edges) with $n>2$ vertices is $3n-6$

It seems sort of related to triangle mesh representations of solids

33. ivan.trifonov says:

$\int_{M}^{ } \,dw = int_{dM}^{ } w\,$

34. Kevin LoGrande says:

Linear/Angular Momentum Balance in Continuum Mechanics
$\nabla \cdot \mathbf{\sigma}+\rho\mathbf{b}=\rho\mathbf{a}$
$\mathbf{\sigma}=\mathbf{\sigma}^T$

35. Ulibos says:

For a vector $V$ it’s Euclidean length $l$ can be found as $l=\sqrt{\sum_{s\in V}^{}s^2}$

36. jiejiao says:

Holder’s inequality:

For $p, q \in [1, \infty)$ with $1/p + 1/q = 1$, and real vectors $f, g$, $$||\langle f, g \rangle ||_1\leq ||f||_p \cdot ||g||_q$$

37. tiffany2 says:

Pythagorean Theorem: simple but I use it often

$a^2 + b^2 = c^2$

38. AY says:

For $u + iv = f(z)$, where $z=x+ iy$, to be a analytical function the partial derivatives condition is given by Cauchy and Riemann eqns:
$u_x = v_y$ and $v_x = – u_y$.

39. amiltonwong says:

\begin{equation}
\begin{aligned}
\end{aligned}
\end{equation}

40. Brian says:

“Let there be light”
\begin{equation}
L_o = L_e + \int\limits_\Omega L_i f_r \cos\theta d\omega
\end{equation}

41. eharber says:

The Euler-Lagrange Equation:

$$\frac{\partial \cal{L}}{\partial q_i} – \frac{d}{dt}\frac{\partial \cal{L}}{\partial \dot{q_i}} = 0$$

42. khg says:

Parkinson’s Law
$\Delta t_\textsf{taken to do work} \leq \Delta t_\textsf{allotted for its completion}$

43. WillF says:

$$\sum_{i=0}^\infty r^i = {1 \over 1 – r}, \vert r \vert \lt 1$$

44. khuser23 says:

$\sum_{k=0}^n {n \choose k}^2 = {2n \choose n}$

45. Diram_T says:

$$\sum_{i=0}^{n} \binom{i}{r} = \binom{n+1}{r+1}$$

46. agoberna says:

$$x = \frac{-b \pm \sqrt{b^{2}-4ac}}{2a}$$

47. karndalmia says:

1-(1/2)+(1/3)-…+… = ln(2)

48. Brad Zhang says:

$$\sum_{i=0}^\infty 2^i= -1$$