Split to two cases:
[x] > x
--->[x] < x+1
(trivial, and I think you agree with it)[x] = x
--->[x]+1 = x + 1
--->[x] < x+1
Similarly for x-1 < floor(x)
, split to two cases:
floor(x) < x
--->floor(x) > x-1
floor(x) = x
--->floor(x) - 1 = x - 1
--->floor(x) > x-1
So, in the last equation - all is left is floor(x)<=c<=ceil(x)
- which is pretty much directly from their definition, and from the above two claims we get that:
x -1 < flooring(x) <= x <= ceiling(x) < x+1