Draw a Venn diagram and decide region by region what belongs to the set expression.
For counting problems, fill in the regions from the centre outwards: all sets, then pairs, then single sets, then the outside.
Use inclusion–exclusion as a check: |A ∪ B| = |A| + |B| − |A ∩ B|.
For a relation, test each property on every pair and give one counterexample when it fails.
For a function, count the arrows arriving at each element of B: at most one means injective, at least one surjective.
Graph theory
Handshake lemma
Σ deg(v) = 2|E|
Complete graphs
|E(Kn)| = n(n − 1)/2, |E(Km,n)| = m·n
Regular graph
|E| = n·k/2
Tree and forest
|E| = n − 1, |E| = n − c
Leaves of a tree
L = 2 + Σ (deg(v) − 2) over deg(v) ≥ 3
Euler
circuit: all degrees even; path: exactly 2 odd degrees
Hamilton circuits in Kn
(n − 1)!/2
Dijkstra update
d(v) = min(d(v), d(u) + w(u, v))
Walks of length L
(Mᴸ)ij
Method
Write down the degree of every vertex; the degrees add up to twice the number of edges.
For Euler questions, check that the graph is connected and count the vertices with odd degree.
For trees, use n − 1 edges and the handshake lemma.
For Dijkstra, keep a table of tentative distances and settle the smallest unsettled vertex each round.
For a minimum spanning tree, sort the edges by weight and skip every edge that closes a cycle (Kruskal), or grow one tree from a start vertex (Prim).
Number theory & modular arithmetic
Division with remainder
a = q·n + r, 0 ≤ r < n
Euclid
gcd(a, b) = gcd(b, a mod b)
Least common multiple
lcm(a, b) = a·b/gcd(a, b)
Inverse
a·a⁻¹ ≡ 1 (mod n), gcd(a, n) = 1
Linear congruence
a·x ≡ b → x ≡ a⁻¹·b (mod n)
Euler’s totient
φ(n) = n·∏(1 − 1/p), φ(pᵏ) = pᵏ − pᵏ⁻¹
Fermat
aᵖ⁻¹ ≡ 1 (mod p)
RSA
n = pq, φ = (p − 1)(q − 1), d = e⁻¹ mod φ, c = mᵉ mod n, m = cᵈ mod n
Method
Reduce every number modulo n first; remainders lie between 0 and n − 1.
For a gcd, divide repeatedly with Euclid’s algorithm; the last nonzero remainder is the gcd.
For an inverse, run the extended algorithm and keep the coefficient of a; add n if it is negative.
Solve a·x ≡ b by multiplying with a⁻¹; combine two congruences by writing x = r₁ + m₁k.
For powers, write the exponent in binary and square repeatedly; in RSA, d = e⁻¹ mod φ(n) with φ(n) = (p − 1)(q − 1).
2Practice problems
1.Propositional logic & proofsMedium
You want to prove ¬r → ¬q by contradiction. Which statement do you assume at the start? Click it.
¬r → ¬q
1¬r ∧ ¬¬q
2¬¬r → ¬¬q
3¬¬r ∧ ¬q
4¬¬r ∧ ¬¬q
2.Propositional logic & proofsHard
Which statement is equivalent to ¬∃x ∀y R(x, y), with the negation moved all the way inside? Click it.
¬∃x ∀y R(x, y)
1∀x ∀y ¬R(x, y)
2∀x ∃y ¬R(x, y)
3∃x ∀y ¬R(x, y)
4∃x ∃y ¬R(x, y)
3.Sets, relations & functionsMedium
A survey of 53 people finds that 26 own a laptop (L), 24 a tablet (T) and 17 a smartwatch (W). 8 own L and T, 12 own L and W, 9 own T and W, and 6 own all three. How many are in L only?
|U| = 53
|L − (T ∪ W)| =
4.Sets, relations & functionsHard
The arrow diagram shows a function f: A → B with A = {1, 2, 3} and B = {a, b, c, d}. Click the description that fits f.
f = {(1, d), (2, d), (3, b)}
1injective, not surjective
2surjective, not injective
3bijective
4neither
5.Graph theoryMedium
How many different Hamilton circuits H does the complete graph K4 have? Circuits that differ only in the starting vertex or the direction count as the same.
K4
H =
6.Graph theoryHard
Kruskal’s algorithm builds a minimum spanning tree. Click the edge it adds as edge number 3; skipped edges do not count.
k = 3
1edge AB
2edge AC
3edge BC
4edge BD
5edge CE
6edge DE
7edge BE
7.Number theory & modular arithmeticMedium
Find the inverse of 23 modulo 89: the number x with 0 < x < 89 and 23·x ≡ 1 (mod 89).
a = 23n = 89
23−1 mod 89 =
8.Number theory & modular arithmeticHard
Encrypt the message m = 726 with the RSA public key (n, e) = (1517, 7). Find the ciphertext c.
n = 1517e = 7m = 726
c =
3Solutions
1.Propositional logic & proofs
Answer: ¬r ∧ ¬¬q
Logical equivalence: two formulas are equivalent (≡) when they have the same value in every row of the truth table. A tautology is 1 in every row, a contradiction is 0 in every row, and a contingency is neither. Useful laws: De Morgan, ¬(a ∧ b) ≡ ¬a ∨ ¬b and ¬(a ∨ b) ≡ ¬a ∧ ¬b; a → b ≡ ¬a ∨ b ≡ ¬b → ¬a (contrapositive); ¬(a → b) ≡ a ∧ ¬b; and the distributive laws, a ∨ (b ∧ c) ≡ (a ∨ b) ∧ (a ∨ c).
Parts of the implication
a = ¬r, b = ¬q
Negation of the implication
¬(a → b) ≡ a ∧ ¬b = ¬r ∧ ¬¬q
Answer
¬r ∧ ¬¬q
If the assumption leads to a contradiction, the implication must be true.
2.Propositional logic & proofs
Answer: ∀x ∃y ¬R(x, y)
Quantifiers and arguments: ¬∀x P(x) ≡ ∃x ¬P(x) and ¬∃x P(x) ≡ ∀x ¬P(x). Move a negation inwards, swap every quantifier it passes, and negate the inner formula with the propositional laws, such as ¬(a → b) ≡ a ∧ ¬b. An argument is valid when the conclusion is 1 in every row in which all premises are 1. A single row with true premises and a false conclusion is a counterexample and shows the argument is invalid.
Move the negation inwards
¬∃x ∀y R(x, y) ≡ ∀x ∃y ¬R(x, y)
Answer
∀x ∃y ¬R(x, y)
The other options either keep a quantifier unchanged, change the order of the quantifiers, or negate the inner formula wrongly.
3.Sets, relations & functions
Answer: |L − (T ∪ W)| = 12
Sets and Venn diagrams: A ∪ B holds the elements in A or B (or both), A ∩ B the elements in both, A − B the elements of A that are not in B, and the complement A′ everything in U that is not in A. In a Venn diagram every set expression is a union of regions, so count it region by region. Inclusion–exclusion: |A ∪ B| = |A| + |B| − |A ∩ B|, and |A ∪ B ∪ C| = |A| + |B| + |C| − |A ∩ B| − |A ∩ C| − |B ∩ C| + |A ∩ B ∩ C|.
Centre and pairs
L ∩ T ∩ W: 6
L ∩ T only: 8 − 6 = 2
L ∩ W only: 12 − 6 = 6
T ∩ W only: 9 − 6 = 3
Single sets
L only: 26 − 2 − 6 − 6 = 12
T only: 24 − 2 − 3 − 6 = 13
W only: 17 − 6 − 3 − 6 = 2
Answer
|L − (T ∪ W)| = 12
Answer
|L − (T ∪ W)| = 12
4.Sets, relations & functions
Answer: neither
Functions and counting: a function f: A → B gives every element of A exactly one image in B. f is injective (one-to-one) when different elements have different images, surjective (onto) when every element of B is an image, and bijective when both hold. With |A| = m and |B| = n there are nᵐ functions, n!/(n − m)! injective ones (m ≤ n), and by inclusion–exclusion Σ (−1)ᵏ·C(n, k)·(n − k)ᵐ surjective ones. |𝒫(A)| = 2ᵐ and |A × B| = m·n.
Injective?
f(1) = f(2) = d → not injective
Surjective?
a, c receive no arrow → not surjective
Answer
neither
Answer
neither
5.Graph theory
Answer: H = 3
Euler, Hamilton and trees: a connected graph has an Euler circuit (every edge once, back to the start) when every vertex has even degree, and an Euler path that is not a circuit when exactly two vertices have odd degree; the path then runs from one odd vertex to the other. A Hamilton circuit visits every vertex once; Kn has (n − 1)!/2 of them. A tree on n vertices has n − 1 edges, and a connected graph with m edges keeps a spanning tree after removing m − n + 1 edges.
Count
H = (4 − 1)!/2 = 6/2 = 3
Answer
H = 3
6.Graph theory
Answer: edge CE
Shortest paths and spanning trees: Dijkstra’s algorithm settles one vertex per round: the unsettled vertex with the smallest tentative distance. Its distance is then final, and every neighbour v gets d(v) = min(d(v), d(u) + w(u, v)). Kruskal’s algorithm builds a minimum spanning tree by taking the edges from light to heavy and skipping any edge that would close a cycle; Prim’s algorithm grows one tree from a start vertex by always adding the lightest edge that leaves it. A spanning tree of n vertices has n − 1 edges.
Kruskal’s algorithm
1. AC (6)
2. BD (8)
3. CE (9)
4. AB (14)
edge number 3
CE
Answer
edge CE
7.Number theory & modular arithmetic
Answer: 23−1 mod 89 = 31
Inverses and congruences: a has an inverse modulo n exactly when gcd(a, n) = 1. The extended Euclidean algorithm writes 1 = s·n + t·a, and then a⁻¹ ≡ t (mod n). A congruence a·x ≡ b (mod n) with gcd(a, n) = 1 has the solution x ≡ a⁻¹·b. Two congruences x ≡ r₁ (mod m₁) and x ≡ r₂ (mod m₂) with coprime moduli have one solution modulo m₁·m₂ (Chinese remainder theorem). Euler’s φ(n) counts the numbers from 1 to n that are coprime to n: φ(n) = n·∏(1 − 1/p).
Extended Euclidean algorithm
Each remainder r is written as r ≡ t·23 (mod 89).
r = 89, t = 0
r = 23, q = 3, t = 1
r = 20, q = 1, t = −3
r = 3, q = 6, t = 4
r = 2, q = 1, t = −27
r = 1, q = 2, t = 31
Inverse
1 ≡ 31·23 (mod 89)
x = 31
check: 23·31 = 713 = 8·89 + 1
Answer
23−1 mod 89 = 31
8.Number theory & modular arithmetic
Answer: c = 347
Modular powers and RSA: compute aᵏ mod n by repeated squaring: write k in binary, square, reduce after every step and multiply the powers whose bit is 1. Fermat: aᵖ⁻¹ ≡ 1 (mod p) for a prime p not dividing a, so the exponent can be reduced modulo p − 1. RSA: choose primes p and q, n = p·q and φ(n) = (p − 1)(q − 1); the public exponent e is coprime to φ(n), and the private exponent is d = e⁻¹ mod φ(n). Encrypt with c = mᵉ mod n and decrypt with m = cᵈ mod n.