Reflexive
Transitive
Symmetric
Inverse function to exponentiation
If odd: middle value
If even: average of the two middle values (1, 2, 3, 4 => (2 + 3) / 2 = 2.5)
From a set of n items, choose k items with 0 <= k <= n
P(n, k)
Order matters: n! / (n - k)! // How many permutations
Order does not matter: n! / ((n - k)! k!) // How many combinations
P(a ∩ b) = P(a) * P(b)
P(a ∪ b) = P(a) + P(b) - P(a ∩ b)
Pb(a) = P(a ∩ b) / P(b)