In a column, write the integer part of the integer multiples of $\displaystyle \sqrt{2}$. Then, in the second column, write the missing integers in increasing order. In the third column, write the difference between the second and first columns. You get :

1 3 2

2 6 4

4 10 6

5 13 8

7 17 10

8 20 12

9 23 14

11 27 16

12 30 18

14 34 20

15 37 22

16 40 24

18 44 26

19 47 28

21 51 30

22 54 32

24 58 34

25 61 36

26 64 38

28 68 40

29 71 42

...

Why does the third column consist of the even integers?