This page describes a go term created by Robert Jasiek.
A dead ko is a ko where one side's stones are dead and can be removed by the opponent quite like ordinary dead stones from territory. Usually such removal occurs only after the endgame.
While regular kos can be or become worth fighting during the game, usually dead kos are and remain uninteresting until after the endgame. On average, regular kos occur in almost every game, dead kos in about every third game, double ko deaths (perpetual kos) about once every 300th game and rare kos (such as double ko seki or triple ko) roughly once every 5,000th game. As the second most frequent ko type, understanding dead ko is much more important than understanding the scarce or rare ko types.
Not every ko with dead strings on or adjacent to it is a dead ko. There are also other ko types having dead strings. In particular, there are double disturbing deaths. While until after the endgame dead kos are not a source for ko threats, double disturbing deaths can be "unlimited" sources for ko threats.
When elsewhere a regular ko fight occurs, then White can make ko threats in the double disturbing death formation.
When elsewhere a regular ko fight occurs, then White does not have any ko threat in the shape having or getting dead kos. Dead kos must not be confused with double kos!
Dead kos must also not be confused with triple kos or greater multiple kos. After the endgame or during scoring, dead kos can be dissolved and the attacker does not need to play like in a triple ko:
The big white string adjacent to the dead stones is also dead: It can be removed.
This concept is relevant
- for strategy or tactics in actual game play because it occurs in about every third game and is as important as recognising non-ko dead strings,
- for a classification of kos,
- to the rules regarding removal of stones after the game has finished, i.e scoring.
A dead ko is a ko by shape, not by actual play. (Some rules resolve questions of life and death by actual play in an encore. But in such cases the possibly dead stones are not removed like ordinary dead stones.)
Each of the 6 kos on the board is a dead ko. Black will be able to remove all the white stones from the board.
This is a counter-example because it is not a ko at all!
This is a dead ko. (Note that each basic ko consists of two intersections.)
Bill: This is a ko only by shape. Even with play White can capture Black without taking the ko.
RobertJasiek: Like every basic ko, it is also a ko by the principle possibility of repetition after 2 plays.
RobertJasiek: For all common restriction rules, this is a position with two examples of dead ko.
A dead ko is a ko for a player so that
- the open player moving first can virtual-force uncapturable life of his on the ko,
- the closed player moving first can virtual-force uncapturable life of his on the ko,
- the open opponent moving first cannot virtual-force uncapturable life of his on the ko, and
- the closed opponent moving first cannot virtual-force uncapturable life of his on the ko.
For details see Types of Basic Kos
For "a player" insert either "Black" (and the opponent is "White") or insert "White" (and the opponent is "Black").
Bill: Because it introduces new terms, this definition "raises more questions than it answers", as they say.
RobertJasiek: It introduces the shortcuts "open player" for "player for whom the ko is open", "closed player" for "player for whom the ko is closed", "uncapturable life" for "two-eye-formation or uncapturable string", and "on the ko" for "on both intersections of the ko". The only term with real new contents is "virtual-force". I think that that term's change from "force" is simpler than the term "ko master" or than abstract environments of ko threats.
RobertJasiek: As long as they are dead kos. As a consequence of ko threats, they might resurrect. Regardless, virtual-force is important in a definition of dead ko also to distinguish it clearly from other ko types. In fighting ko, it is essential. Its consistent usage in all disturbing ko types and in fighting ko allows a nice classification of them.
These restriction rules are representative for all common restriction rules when applied to the example.
Black can continue to create a two-eye-formation that also covers the two initial kos.
Black can remove White also if one or both of the kos is initially open for Black. Contrarily, White cannot create any white two-eye-formation or white uncapturable string on either ko. The summarizing conclusion is that each of the initial kos is a dead ko.
A study under Basic-Fixed Ko Rules and 2 ending passes would yield the same sequences and conclusion as under positional superko and 2 ending passes.
tderz: Above 'dead ko'-definition reads: "A dead ko is a ko where one side's stones are dead and can be removed by the opponent quite like ordinary dead stones from territory. "
Are 'dead ko's a6, d9, f1 and i4 all the same - but not inter-related?
If they are (inter-related), does above definition then suffice? (and encompass all possible cases?)
RobertJasiek: Forget about reading the informal description as if it were a formal definition. - For the purpose of my formal definition, each of the four kos is analysed independently of (possibly not yet) assessed ko types of other kos.
- Moonshine life (a special case of dead ko where the "dead" stones cannot be captured in some rule sets)