There are n-1 possibilities to choose the second element. Permutations Take a set A of n different elements. Choose the n elements in a specific order. Each such choice is called a permutation of n elements. From above we know that such permutation is a variation of n elements, taking all the elements in a specific order.
Combinations Take a set A of n different elements.
Choose a subset of p elements. Such choice is called a combination of n elements choose p. To prove this theorem we use mathematical induction. This proves the theorem. Counting with repetition Variations with repetition Take a set A of n different elements.