13. (8) (Problem 14.55, textbook) (a) Let r be the number of length n binary strings in which 011 occurs starting at the 4th position Write a formula for r in terms of n. (b) Let Ai be the set of length n binary strings in which 011 occurs starting at the ith position. (So Ai is empty for i > n-2.) If i f j,theintersection Ai n Aj is either empty or of size s. Write a formula for s in terms of n coefficient for t in terms of n as an integer or as a simple expression which may include the constants, r, s and t above Hint: (c) Let t be the number of intersections Ai n Aj that are nonempty, where i n-2.) If i f j,theintersection Ai n Aj is either empty or of size s. Write a formula for s in terms of n coefficient for t in terms of n as an integer or as a simple expression which may include the constants, r, s and t above Hint: (c) Let t be the number of intersections Ai n Aj that are nonempty, where i