00:01
Okay, so let's construct and let's show that phi going from f to the power set of s, which takes f to phi of f defined by those elements x belonging to s, such that f of x is equal to 1.
00:23
Well, let's show that this function is a bi -jection.
00:27
Okay, number one, phi is injective.
00:32
Okay, this is easy to prove.
00:35
If we have phi of f equals phi of g, then this thing implies that f of x is equal to 1, if and only if g of x is equal to 1.
00:52
But now we know that f and g are functions going from s to 01.
01:00
So in particular, if f and g are both, if f is equal to one, if and if g is equal to one, well, since the only element left is zero, what do we have? we have that f of x must be identically equal to g of x for every x belonging to s.
01:23
So, as i said, this is easy to understand because f and g are functions going from s.
01:30
To a set with two points only that's why they are determined on the values associated to one okay now number two f is subjective okay oh let me rewrite this way okay subjective well how do we show this if y is a subset of s then we define f going from s to zero one as follows we are going to an element x to one let me separate this one okay one if x belongs to y zero if x does not belong to y.
02:38
Okay, so if x does not belong to y.
02:43
And well, by construction, what we have, we have that phi of f is exactly y...