A journey through Combinatorics: from the Addition and Multiplication Principles
to the Binomial Theorem, by way of permutations, arrangements and combinations,
with and without repetition. The complete roadmap to mastering the art of
counting possibilities.
The articles are listed below, in the order they were written.
How many 8-character passwords are there? In how many ways can 10 people sit around a table? Combinatorics answers without listing every case. This is the roadmap of the series: from the counting rules to permutations, arrangements, combinations and the binomial theorem.
With 4 math books and 3 Portuguese books, there are 7 ways to pick one book and 12 ways to pick one of each. The first count is a sum and the second is a product: these are the sum rule and the product rule, applied here to passwords, flags, answer keys, and counting divisors.
How many anagrams does the word VIRUS have? In how many ways can 5 friends sit around a table? The answers are 120 and 24, by simple and circular permutations. We will also see adjacency restrictions, the traveling salesman problem, and derangements.
In a race with 10 athletes, the podium can be filled in 720 ways. That is an arrangement, an ordered selection of some elements of a set. We solve problems about routes, license plates, and numbers with distinct digits, with Python and SQL.
Out of 10 friends, 3 go buy the popcorn. Only who goes matters, not the order of the choice, so there are 120 possible groups. We will see the combination formula, its symmetry, the complement method, and how to split a set into groups.
How many anagrams does the word MAMBEMBE have? Fewer than an 8-letter word with distinct letters would have, because swapping two equal letters does not create a new anagram. The formula discounts the repetitions by dividing by factorials, and it works for binary sequences and for shortest paths on grids.
A 4-digit password allows repetition, and that is why there are 10,000 of them: ten options in each of the four positions. The arrangement with repetition counts passwords, license plates, and RGB colors, and shows how many patterns fit in the 32 bits of a float and in the registers of an HP-11C calculator.
At an ice cream shop with 7 flavors, there are 210 ways to order 4 scoops, repeating flavors allowed. We get there with the stars and bars method, which shows that choosing with repetition, distributing balls into boxes, and solving integer equations are the same problem.
A set with 10 elements has 1,024 subsets, and that is the sum of row 10 of Pascal’s triangle. We will see the identities of binomial coefficients: symmetry, Pascal’s rule (Stifel’s rule), and the row, column, and diagonal theorems.
What is the coefficient of a term in the expansion of a binomial raised to the tenth power? The binomial theorem answers in one line, with the coefficients from Pascal’s triangle. We will see the theorem and its proof by induction, the general term formula, and the identities that come from special cases.
