Welcome to the **Combinatorics** page. Here you can browse an array of combinatorics topics from permutations and combinations to the binomial and multinomial theorems to chains and antichains and much more!

# Combinatorics Topics

## 1. Sets, Set Operations, and some Important Functions

## 2. Permutations and Combinations

- The Addition and Subtraction Principles
- The Multiplication and Division Principles
- The Pigeonhole Principle
- The Generalized Pigeonhole Principle
- Increasing or Decreasing Subsequences of Finite Sequences
- Techniques for Solving Counting Problems
- Solving Counting Problems with Groupings
- Permutations of Elements in Sets
- Circular Permutations
- Combinations of Elements in Sets
- Counting Lattice Paths

## 3. Pascal's Triangle and The Binomial Theorem

- Pascal's Triangle
- The Fibonacci Sequence
- The Combinatorial Derivation of the Fibonacci Sequence
- A Closed Form of the Fibonacci Sequence
- Pascal's Formula
- Binomial Coefficients
- Binomial Coefficient Identities
- Sums of Binomial Coefficients
- Vandermonde's Identity
- The Formula for the Finite Sum of Consecutive Positive Integers
- The Generalized Binomial Coefficient Formula
- The Binomial Theorem
- Counting the Number of Subsets of a Finite Set

## 4. Combinations from Multisets, The Trinomial Theorem and The Multinomial Theorem

- Permutations of Elements in Multisets with Infinite Repetition Numbers
- Permutations of Elements in Mulitsets with Finite Repetition Numbers
- Combinations of Elements in Multisets with Infinite Repetition Numbers
- The Urn Problem with Indistinguishable Balls
- Nonnegative Integral Solutions to Simple Equations
- Trinomial Coefficients
- Trinomial Coefficient Identities
- The Trinomial Theorem
- Multinomial Coefficients
- Multinomial Coefficient Identities
- The Multinomial Theorem
- Newton's Generalization of The Binomial Theorem

## 5. Chains, Antichains, the Inclusion-Exclusion Principle, and Derangements

- Chains of Subsets of a Set
- Antichains of Subsets of a Set
- The Intersection of a Chain and Antichain
- The Lubell-Yamamoto-Meshalkin Inequality
- Sperner's Antichain Theorem
- The Inclusion-Exclusion Principle
- Combinations of Elements in Multisets with Finite Repetition Numbers
- Restricted Nonnegative Integral Solutions to Simple Equations
- Derangements
- Estimating the Number of Derangements of Elements from a Finite Set
- A Recurrence Relation for the Derangement Numbers

## 6. Algorithms for Generating Permutations and Combinations

- Generating All Permutations of a Set
- Mobile Generating of All Permutations of a Set
- Inversions of Permutations
- Even and Odd Permutations
- Evaluating Determinants with Inversions of Permutations
- The Inversion Sequence of a Permutation
- Generating Permutations with Inversion Sequences
- An Alternative Method for Generating Permutations with Inversion Sequences
- Generating All Combinations of a Set
- Generating k-Combinations of a Set

## 7. Relations on Sets and Partial/Total Orders

- Relations on Sets
- The Empty, Universal, and Identity Relations on a Set
- The Number of Distinct Relations on a Finite Set
- Partial Orders and Strict Partial Orders on Sets
- Linear Extensions of Partial Orders on Finite Sets
- Total Orders on Sets
- Total Orders from Permutations of a Set
- Equivalence Relations
- Equivalence Classes
- Different Representatives for Equivalence Classes
- Hasse Diagrams
- The Matrix Representation of a Relation
- Matrix Representations of Various Types of Relations

## 8. Generating Functions and Systems of Distinct Representatives

- Some Important Infinite Series
- Generating Functions
- More on Generating Functions
- Determining Generating Functions
- Using Generating Functions to Solve Counting Problems
- Positive Integer Partitions
- Exponential Generating Functions
- More on Exponential Generating Functions
- Systems of Distinct Representatives
- Hall's Marriage Theorem

# 9. Recurrence Relations

- Linear Recurrence Relations
- Characteristic Equations of Linear Recurrence Relations
- Vandermonde Matrices
- Distinct Roots - Linear Homogeneous Recurrence Relations with CC
- Solving LHRRs with CCs and Distinct Roots of the Characteristic Equation
- Repeated Roots - Linear Homogeneous Recurrence Relations with CC
- Solving LHRRs with CCs and Repeated Roots of the Characteristic Equation
- Solving Linear Homogeneous Recurrence Relations with Generating Functions

# 10. Special Counting Sequences and the Calculus of Finite Differences

- The Catalan Numbers
- A Recurrence Relation for the Catalan Numbers
- The Difference Operator
- More Properties of the Difference Operator
- Higher Order Differences
- Taylor's Theorem for the Calculus of Finite Differences
- Difference Sequences
- Difference Tables of Sequences
- The Antidifference Operator
- The Fundamental Theorem of the Calculus of Finite Differences

# 11. Designs

- Block Designs
- Balanced Incomplete Block Designs
- The Replication Number of a Balanced Incomplete Block Design
- The Block Number of a Balanced Incomplete Block Design
- Symmetric Balanced Incomplete Block Designs
- The Existence of Balanced Incomplete Block Designs
- The Sum Construction of Balanced Incomplete Block Designs
- The Complement Construction of Balanced Incomplete Block Designs
- The Incidence Matrix of a Balanced Incomplete Block Design
- Basic Properties of Incidence Matrices of Balanced Incomplete Block Designs
- Isomorphisms and Automorphisms of Balanced Incomplete Block Designs

- Difference Sets
- Partial Difference Sets
- The Trivial Difference Sets
- Paley Difference Sets
- The (v, k, λ) Parameters of Paley Difference Sets
- Twin Prime Difference Sets
- Difference Sets in Non-Abelian Groups
- The Development of a Difference Set
- Singer Difference Sets
- The Bruck-Ryser-Chowla Theorem

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