[SkiFire13]

I wonder what's the reason of picking groups of 5 elements instead of 2 or 8.

[danlark]

3 and 4 elements will fail to prove the complexity is linear

You still can do 3 or 4 but with slight modifications

https://arxiv.org/abs/1409.3600

For example, for 4 elements, it's advised to take lower median for the first half and upper median for the second half. Then the complexity will be linear

[lalaland1125]

1. You want an odd number so the median is the middle element of the sublist.

2. One and three are probably too small