Okay.
Welcome back.
Still week six, wow.
One more week to go.
So, here we're in impartial games,
reversible moves and we'll have a few
examples and MEX.
But I think that it's going to be two
pieces.
So, let's take a look at this.
So, let's talk about impartial games.
Impartial game is where like Nim, both
left and right have the same possible
moves.
Not like five which gives five moves to
the left and no moves to right.
Or minus five, which gives five moves to
right but none to left.
Five and minus five are partisan games.
They're games where it depends on whether
you're left or right.
Whereas, Nim is impartial.
In general, a game is called impartial if
every left move is also a right move, and
every right move is a left move.
And every move in every position is also
an impartial game.
So, let's look at an example.
Okay.
Now, when we write down the impartial
games, we don't have to say what the left
moves are and the right moves are.
All we have to do is say what the moves
are.
This is because every left move is a right
move and every right move is a left move.
So, let's look at the game where the moves
are to Nim heap for size zero, size one,
size two, size five or size seven.
Okay.
So, my claim is, is the same as this.
So, so think of this as a Nim heap for
size three, that you can add two coins to
or you can add let's see three, three,
four coins to.
So, so start with start with a Nim heap
for size three, and either you remove one
or more coins or you add two coins or you
add four coins.
That's this game G.
Now if you add two coins, I'll just take
them away and put them in my pocket.
And if you add four coins, I'll just take
the, take, take the four coins and put
them in my pocket.
So the, the move to star five, to five
coins, which you get by adding two coins
and the move to star seven, which you get
by adding four coins, is irreversible.
The other player just takes them away and
pockets them.
And if they're like something really
exotic like a, like a rare year of the
Morgan Dollars or something, it might be
worth something, it might be worth
something.
In, say, grade 65 or so, that would be
worth a lot of money.
So , so, if you add coins, the other
player can just take them away and you're
back to where you started.
What this says is, this game here is the
same as this game here.
But this game here is the same as just the
Nim heap of size 3.
Which you can then move from Nim heap of
size 3 or through a Nim heap of size 2, 1,
or 0.
And we note here that 3 is the, what's
recalled the minimal excluded number out
of 1, 2 and 3.
That is, you look at 1, 2 and 3 and look
at the first natural number that's
missing.
And in general, what happens, so let's
take a look at some examples, of, of Mex.
And we'll leave that for you to compute
for next time.
And then we'll have some, go back to
examples of games.
So the minimal excluded of 1, 3, 2, 1, 17.
Let's see.
One is there, two is there.
I'm sorry.
0 is there, 1 is there, 2 is there, 3 is
there, 4 isn't.
So it's 4.
The minimal excluded of this is, well, 0
is not there.
So, it's 0.
And the minimal excluded of this is 5, if
I did this correctly.
And so here's the minimal excluded for you
to get.
Now, it occurs to me that I didn't do the
problem from last time.
And so let me just leave up some numbers
here.
If we take a Nim heap of size 11, 3, and
10.
11, 13, and 10.
Write them out in binary.
There's one here, go up to the one here.
Circle that.
101, take 101 Nim sum 100.
We get what do we get?
We get 001.
So, the winning move is here.
Just keep the 11 the same.
Take the 13.
And change it to a 1 and take the 10 and
this is the same.
So that's, that's the solution, I believe
to the to the problem at the end of the
last module.
And there we have it, end of another
module.