In mathematics, specifically combinatorial game theory, on, written as on {\displaystyle {\textbf {on}}} , is the value of the loopy game where Left may only loop back the same position, while Right has no moves. on {\displaystyle {\textbf {on}}} may be represented by surreal form on = { on ∣ } . {\displaystyle {\textbf {on}}=\{{\textbf {on}}\mid \}.} Because of Right's inability to move, on {\displaystyle {\textbf {on}}} is positive, as Left automatically wins. Its negative counterpart, off, written as off {\displaystyle {\textbf {off}}} , may be represented by off = { ∣ off } . {\displaystyle {\textbf {off}}=\{\mid {\textbf {off}}\}.}
on {\displaystyle {\textbf {on}}} and off {\displaystyle {\textbf {off}}} are not additive inverses. Their sum, dud (deathless universal draw), written as dud {\displaystyle {\textbf {dud}}} , always loops back to itself, that is, dud = { dud ∣ dud } . {\displaystyle {\textbf {dud}}=\{{\textbf {dud}}\mid {\textbf {dud}}\}.}
dud {\displaystyle {\textbf {dud}}} always results in a draw, hence its name.
on {\displaystyle {\textbf {on}}} is greater than any ender G {\displaystyle G} , as in the difference game on − G {\displaystyle {\textbf {on}}-G} , Left may play solely in on {\displaystyle {\textbf {on}}} , while Right has no option but to play in − G {\displaystyle -G} . Because − G {\displaystyle -G} is an ender, play in − G {\displaystyle -G} will eventually end, and Left will win. Similarly, off {\displaystyle {\textbf {off}}} is less than any ender.
Value- on {\displaystyle {\textbf {on}}} games
Hackenbush In Hackenbush, a position with infinitely many blue edges on the ground has value on {\displaystyle {\textbf {on}}} , as chopping an edge still leaves infinitely many edges remaining. This example illustrates the fact that on + 1 = on {\displaystyle {\textbf {on}}+1={\textbf {on}}} ; adding a blue branch won't change the position.
Col The Col position to the left has value on {\displaystyle {\textbf {on}}} . Right cannot move, while Left may color any of the empty areas, while not changing the position.
Snort A similar Snort position has value on {\displaystyle {\textbf {on}}} , shown to the right. Again, Right cannot move, while Left may color any of the empty areas, which doesn't change the position.
See also Loopy game Surreal number
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