The next number in the sequence is 13112221.

The sequence is called the "look and say" sequence.

You start off with "1" and say "one-one" thus the next number is "11" then we say the numbers out loud "two-ones" therefore the next number will be "21" and so it goes on.

Let's take the last number as another example: "312211" "one-three one-one two-twos two-ones" which translates to "13 11 22 21".

Hope I helped you! :)
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