The number that doesn’t belong to this series is 4.
This sequence is actually the beginning of the Fibonacci series, where each number is the sum of the two preceding ones. The series starts like this:
- 1 (first)
- 1 (second) – 1 + 0 = 1
- 2 – 1 + 1 = 2
- 3 – 1 + 2 = 3
- 5 – 2 + 3 = 5
- 8 – 3 + 5 = 8
- 13 – 5 + 8 = 13
- 21 – 8 + 13 = 21
However, between 3 and 5, we see 4, which does not fit the pattern of adding the two preceding numbers. Thus, 4 is the outlier in this Fibonacci series.