I expect the last index to be 19.
Why? The range is 0 .. 20 inclusive:
[ 6:17:58.02] C:\test>ranges -M=20 -N=3
0 : from 0 to 6 ( 7)
1 : from 7 to 13 ( 7)
2 : from 14 to 20 ( 7)
========================================
21
[ 7:10:34.53] C:\test>ranges -M=20 -N=2
0 : from 0 to 9 ( 10)
1 : from 10 to 20 ( 11)
========================================
21
[ 7:11:36.69] C:\test>ranges -M=20 -N=1
0 : from 0 to 20 ( 21)
========================================
21
Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
"Science is about questioning the status quo. Questioning authority".
In the absence of evidence, opinion is indistinguishable from prejudice.