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Discussion

After studying the proof that the number of partitions of an integer into odd parts is the same as the number of partitions of that integer into distinct parts, Yolanda was beside herself.

Discussion

After studying the proof that the number of partitions of an integer into odd parts is the same as the number of partitions of that integer into distinct parts, Yolanda was beside herself. Do you guys realize what we just did? We showed that two quantities were equal without saying anything about what those quantities actually were. That's really neat, she said. Nobody said anything for a long time, but after some time Dave said There might be other instances where you would want to be able to communicate fully, yet hold back on every last detail. Bob said I don't get it. Alice interjected a comment that was more of question than a statement Do you mean that parties may want to communicate, while maintaining that the conversation did not occur? Carlos added Or maybe they just want to be able to detect whether anyone else was listening. Now Zori was nearly happy. Privacy and security were big ticket items.

Symbols used here

n!
factorial
n × (n−1) × … × 1; the number of orderings of n things. 0! = 1.
\binom{n}{k}
binomial coefficient, "n choose k"
Number of k-element subsets of n things: n!/(k!(n−k)!).
\sum_{k=1}^{n} a_k
summation
Add a_k for k = 1 up to n.
\prod_{k=1}^{n} a_k
product
Multiply a_k for k = 1 up to n.
\emptyset,\ |A|
empty set, cardinality
The set with no elements; the number of elements of A.

Questions people ask

Permutation or combination?

Ask whether order matters. A lock code is a permutation (order matters); a hand of cards is a combination (it does not).

What is a graph in this sense?

Dots (vertices) joined by lines (edges) — not a plot. Road maps, social networks and molecules are graphs; questions like "is there a route" and "how few colours" are graph theory.

خودت امتحان کن

Parts of this page are adapted from Keller & Trotter, Applied Combinatorics (CC BY-SA 4.0). Condensed and re-explained here; errors are ours.

بیشتر در Combinatorics & Graph Theory

The counting principlesPigeonhole principle and inclusion–exclusionBinomial coefficients and Pascal's triangleRecurrences and generating functionsGraphs: vertices, edges, degreesPaths, cycles, trees, Euler and HamiltonColouring and planar graphs