>> Hello, this is games without dice or
cards.
Combinatoriai Game Theory.
I'm Tom Morley.
So let's start by jumping into the water
head first, and let's play a game.
Let me first introduce you to our two
players.
Our two players are called left and right.
These two players alternate.
One goes, what the other one makes a move,
then the first one makes a move, then the
other one makes a move.
Typically, left is associated with the
color blue and you can remember that
because there's an L is blue and right is
associated with red, and you can remember
that, because there's an r in red.
Now, these players are going to alternate,
alternate, play and the 1st 1 that can't
move loses.
So there's no ties.
At each stage of the game there will be
various moves available to one player or
the other player or perhaps both, and
whoever runs out of moves loses.
So let's take a look at something like
this, and So let's start with a little
horsey.
Okay.
So this is a game called Hackenbush, well,
it's a particular position in the game
called Hackenbush.
And you can start in various ways but we
start with this little horsey.
The two players, red and blue, left and
right, take turns, and what they do is cut
one of their edges of their own color.
So if blue starts, he could cut this edge,
and that edge is no longer there.
Now, these, these edges have helium in
them, so and they're attached to the
ground here, which is green.
So if something gets disconnected from the
ground, it floats off and is not available
to either player.
So, for instance maybe, right goes here
maybe left goes here.
Now, at this point, this whole piece here
is still connected to the ground.
But once right moves, it all floats up,
and now left moves, and there's no moves
left for left.
Okay, so, so whenever the two players
alternate, left cuts blue edges, right
cuts red edges.
Any edge that's either cut or no longer
connected to the ground to where the
nutrients are, the nitrogen and the water
and whatever.
Any edges no longer connected to the
ground, floats off and is not available to
either player.
When a player has no edges to cut, the
game ends, that player loses, okay?
We'll see a lot of examples in terms of
Hackenbush as, as the game goes on.
Let's take a look at some very simple
positions in Hackenbush.
Here's the ground.
It's green.
And now we go, I don't know.
Blue, and blue, and maybe a red in
between.
And maybe a red.
And then, say, a blue on top.
Up, now up.
It's okay that these aren't connected, but
they're connected to the ground, so that's
okay.
So, they're there maybe, maybe red goes
first and red says aha, I'm going to cut
this and now these float off and that top
blue end is no longer available for blue.
Blue say, what's the best move for blue?
I don't know offhand, but blue might cut
this, and then red cuts this, and then
blue cuts this, and now red loses.
So that's how that works.
And we'll, we'll look at various
variations of this where we start with,
with, with more complicated pictures and,
and try to analyze in terms of, of who,
who wins, if left goes first, who wins if
right goes first?
Okay.
Let's take a look at another class of
games.
And this is a class of games all played
with coins.
In this case the coins are pennies and
dimes, but and maybe right is pennies,
because the copper in pennies, although,
there's very little copper actually in
pennies looks kind of red.
And then the dimes, say are, are left.
Now both players push coins to the left in
the direction of, of that and the three
games push, shove, and run over have
slightly different rules.
Now you can, you can, you can push a coin
off the edge, off the end, and if it's off
the end, it's no longer in play.
So the two players alternate pushing one
coin to the left one space.
And let's look at let's look at, say, this
position here in push, the first game.
If left pushes this coin over this, this
coin over one, then that pushes also that,
the, the coin next to it over.
So that's, that's called push.
In, in, in shove, when you push a coin
over, everything to the left moves over
one space also.
So with shove, this, this, this coin over
here gets thrown off the cliff.
In run over, what happens is when you
push, say this coin over here one to the
left, you run over that person and that,
that coin disappear from playing.
So you can look at longer positions to
start with, different relations of coins
on the thing.
I think it's not too hard to show.
Whoever has the right most coin will win
the game.
But, but if we have several of these going
at once, then, then things get a little
bit more complicated.
And still what we want to do, even if we
know who, who's, who wins the game what,
what's the best play maybe to, to delay,
going off the cliff as long as possible.
So, so here's four games to play with and
I urge you to start with pictures like
this.
Find someone to play, find, figure out
whether we want to be left or right
doesn't matter.
Draw a picture like this and go play some
games get some coins out, draw pictures
like this.
Play several of these at once and we'll
explain as the course goes by how to do
that more carefully and, and see what see
what looks like good moves and see what
looks like bad moves.
So there's some games to start with and,
let's, continue next time.