用Mathematica统计字母、字符和单词

1、先给出字符串：a = "If we give a paragraph of English text, we want to get the number of times each letter appears, we need to use the function: LetterCounts.This article introduces the usage of this function."

3、出现次数最多的双字母是ti：LetterCounts[a, 2]

5、出现次数最多的字符是空格：CharacterCounts[a]

7、出现次数最多的单词是we、of、the：Reverse[Sort@WordCounts[a]]