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# Which number should come next in the series 1, 1, 2, 3, 5, 8, 13?

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The next number is 21.

This is a famous sequence called "the Fibonacci sequence". The numbers are "the Fibonacci numbers". You generate each number by adding the last two numbers of the sequence, $s_{n - 1} + s_{n - 2} = s_n$ as so:

• $1 + 1 = 2$
• $2 + 3 = 5$
• $5 + 8 = 13$
• $8 + 13 = 21$

The Fibonacci sequence might be in your notes, if you study arithmetic sequences at school. The Fibonacci sequence is a common example of a computational task.