Combinatorial Game Theory
who wins, and by how much
Generator

{2 | 0} + {2 | 0} — where the temperature goes

{2 | 0} + {2 | 0} — where the temperature goes
{2 | 0} + {2 | 0} — where the temperature goes. Three thermographs on one frame: two positions and their sum. The mean of the sum is the sum of the means, every time. The temperature is not: it is bounded by the hottest of the parts and is often far below it, so the number that says how much is at stake in a whole board cannot be got by adding up the parts.

Three thermographs on one frame: two positions and their sum. The mean of the sum is the sum of the means, every time. The temperature is not: it is bounded by the hottest of the parts and is often far below it, so the number that says how much is at stake in a whole board cannot be got by adding up the parts.

7 essays call thermograph-sum. The drawing above is what it returns with no arguments at all; every call below passes it something, because a placement that passes nothing draws whichever member of the family the generator happens to default to rather than the one its essay argues about.

Where it is called

Changing this generator changes every one of these figures.

{2 | 0} + {2 | 0} — where the temperature goes. Three thermographs on one frame: two positions and their sum. The mean of the sum is the sum of the means, every time. The temperature is not: it is bounded by the hottest of the parts and is often far below it, so the number that says how much is at stake in a whole board cannot be got by adding up the parts. Temperature

Two hot fights that add to a cold number

The mean of a sum is the sum of the means, every time. The temperature is not — it is bounded by the hottest part and is often far below it. Two positions each worth fighting over can add to a plain number that neither player wants to touch.

 The thermograph of {5 | 1}. Temperature runs up the page and value across it. Each wall is where a player is willing to move once a tax of that much is charged per move; above the temperature at which they meet, neither wants to move and the position is worth its mean value. The height of the meeting point is what is at stake. The two marks on the base line are the stops — what each player gets by moving first and playing the fight out with no tax charged at all. Values

Where the fight stops

Keep taking the biggest thing on offer until somebody is left facing a number, and the number reached is a stop. Every position has two of them, one for each player moving first, and between them they say how a fight ends — except for the part of it no number can reach.

 What the rule costs. Every sum of three components from a fixed pool, played out twice: once with one side following the rule "move where the stake is largest" and once with both sides evaluating exactly. The rule is not optimal, the gap is bounded, and the bound is the largest temperature on the board. Temperature

A rule with a guarantee

Evaluating a sum of a dozen fights is impossible; following a rule is not. Move where the stake is largest, and over 220 sums of three hot components the rule scores exactly what perfect play scores in 196 of them, is never more than one point behind, and never ends more than the largest single stake below the mean. The rule that is supposed to be different — answer the threat — chose differently in none of the 220.

 Where a sum's temperature actually lands. Every pair drawn from 45 hot positions, with the temperature of the sum set against the larger of the two temperatures. The bound is never broken and it is almost never used: 864 of the 1035 sums sit exactly at the maximum and 160 are frozen. Temperature

How cold a sum of hot games can be

The temperature of a sum is at most the largest temperature in it, and the bound leaves the whole interval below it open. Over 1,035 pairs the sums do not use that interval: 864 sit exactly at the maximum, 160 are frozen outright, and eleven land anywhere in between — every one of them with a component whose wall bends.

 What is left when the copies pair off. For each position, the difference between n copies and n times the mean, reduced to canonical form. The first two are drawn and the last column says what the sequence does after them: half of these settle into a short cycle and the rest produce a new leftover every time, all of them the same bounded size. Temperature

What is left when the copies pair off

A pile of n copies stays within a bounded distance of n times the mean, and the distance never grows. The difference is a game rather than a number, and what it actually is has a much better answer: for a plain switch it alternates between one fight and nothing at all, and for a fight with a follow-up it is different every time — bounded in size and unbounded in complexity.

 Adding two thermographs. Every pair drawn from the 15 values born by day two that are not numbers — 120 sums — with the two walls added pointwise and compared against the true diagram of the sum. The added walls are always an outer bound and the means always add; the whole diagram is right for 92 of the 120, and the 28 it is wrong for are exactly the pairs in which both components are hot. Temperature

When two thermographs can be added

The temperature of a sum is not the sum of the temperatures, and the natural repair is to add the whole diagrams instead. Over every pair of hot values born by day two the added walls always bound the true ones and the mast always comes out right — and the whole diagram is right exactly when at most one of the two components is hot, which is precisely the case a reader has no use for.

 Where the two diagrams part. Pairs of hot day-two values with the true thermograph of their sum against the one made by adding the components' walls. Every pair parts, and every pair parts at the lower of the two temperatures. Temperature

The second bend is the boundary

Adding two thermographs wall by wall gives a diagram that is right at the mast and wrong below it. Over every pair of hot values born by day two, the added walls sit outside the true ones at every height — an outer envelope with the truth somewhere inside — and the two pictures separate at exactly the lower of the two temperatures, on all twenty-eight pairs. Above that height both components are still fights and the addition is exact; one sixteenth below it, every pair has parted.

The whole library · The position index · The figures that play back