Introduction
The lesson video is the following
Main position
In this lesson we have seen how to invade in the following situation.
There are two main variation depending on the color of the stone on the marked intersection.
White is strong outside
In the case white is strong outside, the san-san reduction as shown below is the only option.
White takes a risk
White can take a risk and play up to 17, trying to kill everything.
It is however a quite risky ko, white could lose the whole corner.
Black is strong outside
When black is strong there are two options to invade. To go high and potentially get the outside, or to go deep and break.
Go high
In this case, white can either defend the corner with B, giving black the outside, or can block the outside with A, leaving black connect easily.
Block the outside
If white protects the outside, then black can easily connect back.
Block the corner
If white defends the corner, then black can take the outside with the move up to 9.
Go deep
If black goes deep, white can protect the corner with A or try to attack strongly with B.
Protect the corner
In this case, white let black connects and keep the corner.
It is important to note that black playing 3 at 4 would die.
White fights
If white tries to fight and cut black, black can live in the corner.
The key moves are the combination of 3 and the cool tesuji 5. After that, black can easily live.
White mistake
If white tries to kill by playing 6, then black can play the following sequence.
After white 16, black can capture at A and can either connect with the triangle intersection or make to eyes playing at 10.
Black mistake
The move at 5 seems natural for black, but it leads to his death.
White 10 is a key move, if white atari the 5 stone, then black can live.