The following number is "1113213211" Why?
The digits are divided into pairs. The first digit of each pair shows how many times we meet the number of the second digit on the number above. So, since the first number is "1", the second is "11" because the first has only one "1", the third is "21" because the second has two "2", the third is "1211" because the second has one "2" one "1" and so on...
So, the last number has one "1", then one "3", then another two "1", then three times "2" and finally another two "1"...