It is a combination of two sequence of + 1 and + 2 addition

```
0 , 0 , 1 , 2 ,2 ,4 ,3 ,6 ,4
adding +1 ;;; 0 , 1 , 2 . 3 , 4 and adding + 2 :: 0 , 2 , 4 , 6 . , so next term is 8 ;;;
all even numbered terms all odd numbered terms
```