COSMO · STUDY + TEACHING DECK

Networks

December 13, 2025

A question worth sharing

Planar Graphs

Use faces and Euler's formula to prove that some networks cannot be drawn without crossings.

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Today’s route

A question becomes an explanation.

Try

Enter through a problem. A partial attempt counts.

Connect

Use planar, face, Euler formula to name what you notice.

Convince

Build an argument a partner can check.

The goal is a method someone else can use.

The opener

Start with a question.

Opening problem

A connected planar graph has V=8 and E=12. How many faces?

Think · 2 min

Sketch, test a small case, or make a first guess.

Pair · 3 min

Compare methods. Find one thing you agree on and one question.

Be ready to share your route, even if it is unfinished.

Compare before checking

Put two attempts in conversation.

Explain

Tell a partner how you started. Let them finish before responding.

Question

Which step feels certain? Which step still needs evidence?

Build

Combine one useful idea from each attempt into a new route.

Can you explain your partner’s method in your own words?

Board conversation

Notice before naming.

Look for structure

Try a small case and record every step.
Separate what changes from what stays fixed.
State the pattern in a sentence before naming the theorem.

Make a conjecture

We think ___ because ___. A case that might break this is ___.

What changed? What stayed the same?

Useful language

Now the language has a job.

planar

drawable without edge crossings

face

region cut out by an embedding

Euler formula

V-E+F=2 for connected planar graphs

Use one of these words to explain something you already noticed.

The core idea

Keep the idea. Explain the conditions.

V-E+F=2

The idea

Use faces and Euler's formula to prove that some networks cannot be drawn without crossings.

What to remember

Faces convert a picture into an equation strong enough to prove impossibility.

What information does this method keep?

Worked example

One route, one decision at a time

The problem

Use Euler's formula to check a cube graph.

Predict each next step

A cube graph has V=8 and E=12.
Its planar drawing has F=6.
Compute 8-12+6.
The result is 2.

Which step needs a reason, not just a calculation?

See the structure

The structure behind planar graphs

V-E+F=2

For a connected planar graph, count the outside region as a face too.

Read the representation

Use faces and Euler's formula to prove that some networks cannot be drawn without crossings.

Look together

Point to the part that explains the relationship. Change one input and predict what changes.

What can you see here that was harder to see in words?

Two approaches

Compare the reasoning.

Structural method

Name the objects and conditions.
Use a reusable relationship.
Explain why the method must work.

Brute force

May help with a small test case.
Can hide the general reason.
Becomes unreliable when the scale grows.

Which route can you justify? What would you test first?

Reasoning clinic

Find the step that needs repair.

Investigate these concerns

May help with a small test case.
Can hide the general reason.
Becomes unreliable when the scale grows.

Repair together

Name the missing condition. Try a small counterexample. Rewrite one step so it is justified.

A useful correction explains why the original step fails.

Work together

Investigate planar graphs

Your shared task

Groups redraw networks to remove crossings, then use Euler's formula when redrawing fails.

Explore

4 min - test cases

Compare

5 min - form a rule

Defend

4 min - defend the rule

Everyone should be able to explain the group’s final claim.

Share → connect

What did the room discover?

Bring your evidence

Share a conjecture, a useful example, or a route that changed.

Connections to investigate

Faces convert a picture into an equation strong enough to prove impossibility.
A complete solution names the condition that makes the method valid.
A counterexample is useful when it exposes a missing condition.

Which observation explains the mechanism?

Try → explain → check

Your turn to build the route

The problem

Show that a connected simple planar graph with V≥3 has E≤3V-6.

Predict each next step

Every face boundary walk has length at least 3; a bridge is counted twice.
Counting edge-sides gives 3F ≤ 2E.
Combine with V-E+F=2.
Rearrange to obtain E≤3V-6.

Which step needs a reason, not just a calculation?

Convince a skeptic

Turn a pattern into a proof.

The claim

$K_{5}$ is not planar.

Build the argument

$K_{5}$ has V=5 and E=10.
A simple planar graph with five vertices has at most 3(5)-6=9 edges.
Ten exceeds the planar bound.

What does each step rely on? Where are the conditions used?

Stretch together

Take the idea somewhere new.

Transfer challenge

Prove that K_(3,3) is not planar using the stronger bipartite face bound.

Choose a route

Name the objects, the conditions, and a relationship that might help. Start with one test case.

What transfers from the earlier example? What is different?

Peer review

Check the argument together.

Restate

Explain the other person’s route before evaluating it.

Test

Try a small case or boundary case. Does every step still make sense?

Revise

Fix an assumption, add a reason, or find a second method.

Where did your thinking change?

Beyond this problem

Where might this idea help?

A connection

Planarity matters in circuit layout, map design, visualization, and network topology.

Discuss the model

What objects or quantities are represented?
Which relationship from today is preserved?
What assumption would you check before trusting it?

Invent one more situation with the same structure.

Exit ticket

Leave with an explanation.

Question 1

A connected planar graph has V=8 and E=12. How many faces?

Question 2

Explain why this claim is true: $K_{5}$ is not planar.

Question 3

Prove that K_(3,3) is not planar using the stronger bipartite face bound.

Choose one. Write an answer with a reason, then compare with a partner.

Reflect together

Make your thinking visible.

I used to think…

Name a claim or approach that changed.

Now I can explain…

Give the reason, not only the result.

I still wonder…

Write a question worth bringing to the next circle.

Share one sentence. Ask one follow-up question.

The idea to keep

Faces convert a picture into an equation strong enough to prove impossibility.

Use the matching practice sets to move from direct examples to proof and transfer.

A good question belongs to everyone in the room.