What is the next number in the sequence 1123581321?

The next number in this well-known Fibonacci Series of numbers is 34. It is usually written with a space and /or comma between the terms, and starts with one, i.e. 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89,...

The next number in the sequence is found by adding the previous 2 numbers together:

for any n where in is a natural number: F(n+1) = F(n) + F(n-1)

So:

1 + 1 = 2

1 + 2 = 3

2 + 3 = 5

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13 + 21 = 34

etc...