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Colouring and planar graphs

Chromatic number, Euler's formula V − E + F = 2, and the four-colour theorem.

Colour vertices so neighbours differ; the fewest colours needed is the chromatic number. For a planar graph V − E + F = 2, which bounds edges by 3V − 6 and shows K₅ is not planar. Picture it: any map needs at most four colours — proven with a computer in 1976. Think it: Euler's formula is the Euler characteristic, the same invariant that distinguishes a sphere from a torus.

ნამდვილი ასლი: 6 - 2

Arithmetic · Arithmetic

Evaluate 6 - 2

4

ჟრყოკა ოჲ ჟრყოკა.

  1. -2 + 6 = 4

    Add: -2 + 6 = 4.

ჲრკპთირვ ჲრდჲგჲპა.
4

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.

How to: Colouring and planar graphs

  1. Add: -2 + 6 = 4.

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.

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მეტი 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 Hamilton