Where on earth the empty string comes?How many bit strings with length not exceeding n, where n is a positive integer, consist entirely of 1s?
Answer is n+1 (counting the empty string)
Once one accepts that the concept of an empty string is quite reasonable, the next question concerns the answer to the exercise in question. Now consider: are all characters contained in the empty string equal to 1? - The answer is yes, of course. Hence the empty string has to be included in the counting of all strings of length consisting entirely of 1s.