
Mathematics for Master Students
Which tasks gave you trouble? Bring them up now: we take the time to go through them before we move on to today’s topic.
Question
Negate: “Some delivery route is unprofitable.”
“Every delivery route is profitable.”: negation swaps \(\exists\) into \(\forall\) and negates the inside.
Common Mistake
“Some route is profitable” is not the negation: both statements can easily be true at the same time.
A set is an unordered collection of distinct objects, called its elements.
Think of a set as a bag: what matters is only what is inside, not in which order it went in, and never twice.
Membership is the most basic set statement:
Every “is it in or not?” question has a clear yes/no answer: that is what makes a set well-defined.
Two standard ways to write a set down:
Set-builder notation is a filter: start from a base set, keep only the elements passing the condition, exactly what a database query does.
The same filter pattern appears throughout your courses:
Every set-builder expression has the same three parts: a base set, a colon, and a condition. Change the condition, and you have a new filter.
Question
How many elements does \(\;\{x \in \mathbb{Z} : x^2 \leq 4\}\;\) have? Shout the number.
Five: the set is \(\{-2, -1, 0, 1, 2\}\).
Common Mistake
Answering \(3\) (only \(0, 1, 2\)) forgets that the base set is \(\mathbb{Z}\): negative integers square to positive numbers, so \(-1\) and \(-2\) pass the filter too.
The empty set \(\emptyset = \{\}\) contains no elements at all.
Common Mistake
\(\emptyset \neq \{0\}\): the set containing zero has one element. The empty set has none.
\(A \subseteq B\): every element of \(A\) is also in \(B\) (“\(A\) is a subset of \(B\)”).
\(\subseteq\) works like \(\leq\) and \(\subset\) like \(<\): the extra line allows equality.
Let \(F = \{x \in \mathbb{N} : x \text{ is divisible by } 4\}\) and \(E = \{x \in \mathbb{N} : x \text{ is even}\}\).
To prove \(A \subseteq B\), argue for an arbitrary element. To disprove it, one counterexample suffices: Session 1’s \(\forall\) versus \(\exists\) again.
Two sets are equal if they contain exactly the same elements:
\[A = B \quad \Leftrightarrow \quad A \subseteq B \text{ and } B \subseteq A\]
The classic confusion: \(\in\) relates an element to a set, \(\subseteq\) relates two sets.
Let \(A = \{1, 2, 3\}\):
| Statement | True? | Why |
|---|---|---|
| \(2 \in A\) | ✓ | \(2\) is an element of \(A\) |
| \(\{2\} \subseteq A\) | ✓ | every element of \(\{2\}\) is in \(A\) |
| \(\{2\} \in A\) | ✗ | the set \(\{2\}\) is not one of the three elements |
Common Mistake
Writing \(2 \subseteq A\): a number is not a set, so \(\subseteq\) does not apply. Ask first: element or set?
Question
Let \(B = \{5, 10, 15\}\). Which statements are true: \(\quad 10 \in B, \quad \{5, 15\} \subseteq B, \quad \{10\} \in B, \quad \emptyset \subseteq B\)?
The cardinality \(|A|\) is the number of elements in \(A\):
Common Mistake
Same bars, different meaning: \(|-5| = 5\) is an absolute value (of a number), \(|\{-5\}| = 1\) is a cardinality (of a set). The argument decides.
Question
What is \(\;|\{1, 2, 2, 3, \{4, 5\}\}|\;\)?
| (a) \(3\) | (b) \(4\) |
| (c) \(5\) | (d) \(6\) |
(b): the elements are \(1\), \(2\), \(3\) and the set \(\{4, 5\}\), four in total.
Common Mistake
The repeated \(2\) counts once (no duplicates in a set), and \(\{4, 5\}\) is one element, not two: it is a set sitting inside a set.
Session 1 was full of sets, we just did not call them that:
Set operations play out inside a universal set \(U\), all objects currently under discussion:
A Venn diagram shows sets as circles inside the rectangle \(U\):

Overlap = shared elements, outside the circles = the rest of \(U\): a thinking tool, not just a picture.
\(A \cup B = \{x : x \in A \text{ or } x \in B\}\): everything in at least one of the sets.

\(\{1, 2\} \cup \{2, 3\} = \{1, 2, 3\}\): the “or” is inclusive, just like in logic; the shared \(2\) appears once.
\(A \cap B = \{x : x \in A \text{ and } x \in B\}\): only the elements in both sets.

\(\{1, 2\} \cap \{2, 3\} = \{2\}\): think of it as applying two filters at once.
\(A \setminus B = \{x : x \in A \text{ and } x \notin B\}\): what is left of \(A\) after removing everything in \(B\).

\(\{1, 2, 3\} \setminus \{2, 3\} = \{1\}\), and beware: \(A \setminus B \neq B \setminus A\) in general, order matters.
\(A^c = \{x \in U : x \notin A\}\): everything in \(U\) outside of \(A\), i.e. \(A^c = U \setminus A\).

Books also write \(\overline{A}\) or \(A'\): same idea. A handy identity: \(A \setminus B = A \cap B^c\).
Let \(U = \{1, 2, \dots, 8\}\), \(A = \{1, 2, 3, 4\}\) and \(B = \{3, 4, 5, 6\}\):
| Expression | Result | Reading |
|---|---|---|
| \(A \cup B\) | \(\{1, 2, 3, 4, 5, 6\}\) | in \(A\) or in \(B\) |
| \(A \cap B\) | \(\{3, 4\}\) | in both |
| \(A \setminus B\) | \(\{1, 2\}\) | in \(A\), not in \(B\) |
| \(B \setminus A\) | \(\{5, 6\}\) | in \(B\), not in \(A\) |
| \(A^c\) | \(\{5, 6, 7, 8\}\) | in \(U\), not in \(A\) |
Note \(A \setminus B \neq B \setminus A\), and \(A^c\) needs \(U\): without a universal set, “everything not in \(A\)” is undefined.
Question
Let \(U = \{1, 2, \dots, 10\}\), \(A = \{2, 4, 6, 8, 10\}\) and \(B = \{1, 2, 3, 4, 5\}\). Shout: \(\;A \cap B, \quad A \setminus B, \quad B^c\), and the number \(|A \cup B|\).
\(A\) and \(B\) are disjoint if \(A \cap B = \emptyset\), no shared elements:

Disjoint sets are how we model clean segmentations: every customer is in exactly one region, no overlaps.
Question
For any two sets \(A\) and \(B\): is \(\quad A \cup B \subseteq A \quad\) always true?
Adding \(|A| + |B|\) counts the intersection twice, so subtract it once, the inclusion-exclusion rule:
\[|A \cup B| = |A| + |B| - |A \cap B|\]
Survey example: of 100 coffee drinkers, 70 take sugar, 60 take cream, 50 take both:
\[|A \cup B| = 70 + 60 - 50 = 80\]
So \(100 - 80 = 20\) drink their coffee black: the complement \((A \cup B)^c\).
Question
Of \(200\) customers, \(120\) ordered online, \(90\) ordered in store, and \(40\) did both. How many customers ordered nothing this year? Shout the number.
\[|A \cup B| = 120 + 90 - 40 = 170, \qquad \text{so } 200 - 170 = 30 \text{ ordered nothing.}\]
Common Mistake
\(120 + 90 = 210 > 200\) is not a contradiction: the \(40\) two-channel customers were counted twice. Subtract the intersection before taking the complement.
Complement flips union into intersection, and vice versa:
\[(A \cup B)^c = A^c \cap B^c \qquad\qquad (A \cap B)^c = A^c \cup B^c\]
Business example: \(A\) = delayed shipments, \(B\) = damaged shipments.
“Neither delayed nor damaged” \(= (A \cup B)^c = A^c \cap B^c\) = “on time and intact”.
This is exactly Session 1’s law \(\neg(p \vee q) \Leftrightarrow \neg p \wedge \neg q\), dressed in set notation.
Set operations are logic on membership statements, \(x \in A \cup B\) means \(x \in A \vee x \in B\):
| Logic (Session 1) | Sets (today) |
|---|---|
| \(p \vee q\) (or) | \(A \cup B\) (union) |
| \(p \wedge q\) (and) | \(A \cap B\) (intersection) |
| \(\neg p\) (not) | \(A^c\) (complement) |
| \(p \Rightarrow q\) (implies) | \(A \subseteq B\) (subset) |
If you remember the logic rule, you get the set rule for free, and the other way around.
Question
Let \(A\) = shipments on time, \(B\) = shipments complete. Which set is “shipments that are late or incomplete”?
| (a) \(A^c \cup B^c\) | (b) \(A^c \cap B^c\) |
| (c) \((A \cup B)^c\) | (d) \(A \cap B\) |
(a): late is \(A^c\), incomplete is \(B^c\), and “or” is \(\cup\). By De Morgan this equals \((A \cap B)^c\): everything that is not “on time and complete”.
Common Mistake
An ordered pair \((a, b)\) is not a set. Here, order does matter:
Curly braces \(\{\,\}\) = unordered set, round parentheses \((\,)\) = ordered pair. The bracket type carries the meaning.
\(A \times B = \{(a, b) : a \in A, b \in B\}\): all ordered pairs with the first entry from \(A\), the second from \(B\).
Example: \(A = \{1, 2\}\) and \(B = \{x, y\}\):
\[A \times B = \{(1, x), (1, y), (2, x), (2, y)\}\]
Counting: every choice from \(A\) combines with every choice from \(B\), so
\[|A \times B| = |A| \cdot |B|\]
Rows from \(A = \{1, 2\}\), columns from \(B = \{x, y\}\): one ordered pair per cell.
| \(x\) | \(y\) | |
|---|---|---|
| \(1\) | \((1, x)\) | \((1, y)\) |
| \(2\) | \((2, x)\) | \((2, y)\) |
Let \(W = \{1, 2\}\) be warehouses and \(C = \{1, 2, 3\}\) be customers:
Question
A carrier runs \(4\) depots and serves \(5\) customers. How many depot-customer routes exist? And if each route can be driven by one of \(2\) vehicle types, how many route-vehicle combinations? Shout both numbers.
The power set \(\mathcal{P}(A)\) is the set of all subsets of \(A\), including \(\emptyset\) and \(A\) itself.
Example: \(A = \{1, 2\}\):
\[\mathcal{P}(A) = \{\emptyset, \{1\}, \{2\}, \{1, 2\}\}\]
Common Mistake
The elements of \(\mathcal{P}(A)\) are themselves sets: \(\{1\} \in \mathcal{P}(A)\), but \(1 \notin \mathcal{P}(A)\).
Question
Let \(A = \{a, b\}\). Which statement is false?
| (a) \(\emptyset \in \mathcal{P}(A)\) | (b) \(\{a\} \in \mathcal{P}(A)\) |
| (c) \(a \in \mathcal{P}(A)\) | (d) \(A \in \mathcal{P}(A)\) |
(c): \(a\) is an element of \(A\), not a subset of \(A\). Only sets live in \(\mathcal{P}(A)\), and \(\{a\}\) is the set that does.
\[|\mathcal{P}(A)| = 2^{|A|}\]
Question
A carrier may serve any subset of the regions \(\{\text{North}, \text{East}, \text{South}\}\). How many different service portfolios are possible?
\(2^3 = 8\), the elements of \(\mathcal{P}(\{\text{N}, \text{E}, \text{S}\})\):
\[\emptyset, \{\text{N}\}, \{\text{E}\}, \{\text{S}\}, \{\text{N,E}\}, \{\text{N,S}\}, \{\text{E,S}\}, \{\text{N,E,S}\}\]
Note that \(\emptyset\) counts: “serve no region at all” is a valid (if unambitious) portfolio.
Let \(P\) = premium customers, \(C\) = churned customers, \(N\) = newsletter subscribers:
AND/OR/NOT are \(\cap\), \(\cup\), \(^c\)A partition splits \(U\) into disjoint pieces that together cover everything:
Overlapping segments (premium and churned) need inclusion-exclusion; a partition is exactly the case where the correction term vanishes.
How to read \(\;(P \cap C) \setminus N\;\)? Work inside out:
Common Mistake
When \(\cup\) and \(\cap\) mix, grouping changes the set: \(A \cup (B \cap C) \neq (A \cup B) \cap C\) in general: when in doubt, add parentheses.
You do not need to compute anything to use set notation: most of its value is in making such statements precise.
Question
\(P\) = premium, \(C\) = churned, \(N\) = newsletter subscribers. Which expression describes “active premium customers who subscribe to the newsletter”?
| (a) \(P \cap C^c \cap N\) | (b) \(P \cup C^c \cup N\) |
| (c) \((P \cap C) \setminus N\) | (d) \(P \setminus (C \cap N)\) |
(a): premium and not churned and subscribed, three filters at once, so three intersections.
Common Mistake
Every constraint defines a set of allowed values, and constraints combine by intersection:
Question
Capacity allows \(q \in [0, 500]\), the supplier requires \(q \in [20, \infty)\). A new customs rule caps single orders at \(15\) units: \(q \in (-\infty, 15]\). What is the feasible set now?
\[[0, 500] \cap [20, \infty) \cap (-\infty, 15] = [20, 500] \cap (-\infty, 15] = \emptyset\]
The problem is infeasible: no order size satisfies all three rules. One constraint has to go, and set notation tells you which pair collides (\(q \geq 20\) against \(q \leq 15\)).
That’s it for today.
Any Questions?
Draw a Venn diagram whenever a set expression looks confusing: two circles resolve most doubts faster than staring at symbols.
See you there, and bring your questions!
Session 02 - Sets | Dr. Tobias Vlcek | Home