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==Introduction to Combinatorics==
===Department of Mathematics, University of Toronto, Fall 2015===

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===Text===
Our main text book will be ''Applied Combinatorics'' (sixth edition) by Alan Tucker, ISBN 978-0-470-45838-9; it is a required reading.

===Further Resources===

* [http://www.math.toronto.edu/cms/undergraduate-program/ Undergraduate Information] at the [http://www.math.toronto.edu/ UofT Math Department]

* [http://www.artsandscience.utoronto.ca/ofr/calendar/crs_mat.htm Undergraduate Course Descriptions].

* [http://www.math.toronto.edu/courses/344.old/ 1999 class], by Steve Tanny.

* [http://www.math.toronto.edu/courses/344/ 2002 class], by Andres Del Junco.

* [http://www.math.toronto.edu/raghavan/344/  2008 class], by Dilip Raghavan.

* My {{Pensieve Link|Classes/15-344-Combinatorics/|15-344 notebook}}.

* [http://drorbn.net/bbs/show?prefix=15-344 Some blackboard shots].

{{15-344:Dror/Students Divider}}


'''(1).''' If you took [http://drorbn.net/?title=12-267 MAT267] (also taught by Professor Bar-Natan), you may remember that Catalan number was discussed in a lecture.

[http://drorbn.net/dbnvp/12-267-121106.php Catalan Number Revisited: Power Series, Combinatorial Information and Ordinary Differential Equations]


'''Homework Problems Revisited: Explore the Textbook'''

(Originally posted in the homework sectors after the corresponding homework was assigned, but moved to here for the readers' convenience.)


'''(2).''' For Problem 10, Section 2.3, you may wonder how to untangle the knot without induction. Here is a possible way.

[[Media:15-344-Solution For Circle Coloring Problem.pdf|Solution to Circle Coloring Problem (Tong Lin)]]


'''(3).''' In Problem 85, Section 5.2, we were asked to find the number of triangles formed by pieces of diagonals or outside edges of an n-gon, assuming throughout that no three lines were collinear. What if there is no such constraint? Attached are two papers that presented some general results.

[https://cs.uwaterloo.ca/journals/JIS/sommars/newtriangle.html 15-344/The Number of Triangles Formed by Intersecting Diagonals of a Regular Polygon]

[http://www-math.mit.edu/~poonen/papers/ngon.pdf 15-344/The Number of Intersection Points Made by the Diagonals of a Regular Polygon]


'''(4).'''In problem 29, Section 5.3, we were asked to prove that 

<math>\sum_{k_1 + k_2 + k_3 = 10} P(10, k_1, k_2, k_3) = 3^{10}</math>, where <math>k_1, k_2, k_3 \in \mathbb{Z}_{\geq 0}</math>.

It is tempting to think that this is a special case of some identity and that a general pattern can be found.

Replacing <math>3</math> with any <math>m \in \mathbb{Z}_{>0}</math> and <math>10</math> with any <math>n \in \mathbb{Z}_{\geq0}</math> gives

<math>\sum_{k_1+k_2+\cdots+k_m=n} P(n, k_1, k_2, \ldots, k_m) = m^n</math>

Does it hold, or can it be generalized to a greater extent?

Yes. We have the [https://en.wikipedia.org/wiki/Multinomial_theorem Multinomial_theorem].

<math>(\sum_{i=1}^m x_i)^n = \sum_{k_1 + k_2 + \cdots + k_m = n} P(n, k_1, k_2, \ldots, k_m) \prod_{j=1}^m x_{j}^{k_{j}}</math>, where <math>m \in \mathbb{Z}_{>0}, n \in \mathbb{Z}_{\geq0}</math> and <math>x_1, \ldots, x_m\in \mathbb{R}</math>.

For a proof by induction, see [https://proofwiki.org/wiki/Multinomial_Theorem here].


'''(5).''' If you are reading section 7.1 in our textbook, you may find example 3 intriguing. Attempting to solve the problem without looking for a recurrence relation, I       found a solution using graph theoretic tools (to be precise, Euler's formula). 

[[Media:15-344-Plane Division.pdf | Graph Theory Revisited: Euler's Formula and a Combinatorial Problem (Tong Lin)]]