A recurrence relation defines each term of a sequence in terms of previous terms — a natural way to describe patterns that build on themselves, like the famous Fibonacci sequence.
1-A 2-A 3-A 4-A | 5 previous 6 21 7 = 21 (5, 9, 13, 17, 21) 8 = 15 (1, 3, 7, 15) 9 = 21 | 10 = 10 (20, 18, 16, 14, 12, 10)