Incursion Go

In this Go variant, standard ko rules are not used. Instead, it is illegal to make an incursion if your opponent's last move was an incursion.

An incursion is a placement by one player in a territory owned by the other player. A territory is an empty point along with all points one can reach from it through a series of steps onto orthogonally adjacent empty points. You own a territory if all stones orthogonally adjacent to it are of your color.

All other rules are the same as in Go. Suicide of one or more stones is not allowed, and area scoring is used.

All known forced Go cycles are impossible in this variant. Unlike the related variant Stoical Go, Incursion Go preserves snapbacks.

If you place a stone on your turn and, by the end of it, the board position is the same as the board position at the end of any of your previous turns, the game ends in a draw. However, if forced cycles are impossible, this will require cooperation between the players, which should not occur in actual play.

In a more radical, less restrictive implementation of this idea, an incursion is said to be minimal if it captures exactly zero or one groups, and it is illegal to make a minimal incursion if your opponent's last move was a minimal incursion. Non-minimal incursions are always legal. This also prevents all known forced cycles.

Luis Bolaños Mures designed Incursion Go in April 2023.

Examples

Eternal life, a Go cycle, is analogous to a dead ko in Incursion Go: Since is an incursion, Black cannot reply with another incursion at the vital point a, but must make a ko threat first, which must itself not be an incursion. Once Black runs out of ko threats, the black group will die.

Moonshine Death: In Go, the white group in the top left corner is alive because White has an unlimited number of unremovable ko threats in the top right corner. In Incursion Go, both corners are proper dead kos for White because and those ko threats are all incursions.

 

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Effects on ko threats

A simple ko  

fractic: This seems to have weird effects on ko threats. In this diagram Black just took the half-point ko at . White would like to use a as a ko threat, but since it is an incursion it would not be allowed.

A simple ko  

fractic: However in this diagram White can play as a ko threat since now Black doesn't own the territory thanks to .