Discrete Mathematics & Logic

Which of the following is a valid propositional logic connective?

How many different permutations are there of the letters in the word $\text{LOGIC}$?

Which counting principle is best used to determine the number of ways to select $k$ elements from a set of $n$ elements where order does not matter?

The empty set is a subset of every set.

Mathematical induction can be used to prove statements about all natural numbers.

In graph theory, a graph with no cycles is called a tree only if it is also connected.