# [FOM] Coins and Infinite Ordinals

Dmytro Taranovsky dmytro at mit.edu
Sun Jul 20 13:51:32 EDT 2014

```General knowledge of infinite ordinals and ordinal notation systems has
been hampered by their complexity and difficulty of visualization.  Here
I present an easily visualizable description using something that most
people know in connection with finite ordinals -- coins.

The general framework is that one's wealth (which represents an ordinal)
is a finite set of coins.  A coin can be exchanged with (equivalently,
is more valuable than) any finite number (including zero) of less
valuable coins.  Thus, a coin corresponds to an ordinal that is a power
of omega, and comparison of wealth reduces to comparison of coins.
(Coins in other denominations are not discussed here.)  Well-foundness
means that if you keep spending and exchanging coins with less valuable
ones without replenishing, then you will eventually run out of money.

-- Up to epsilon_0 --

Description:
* A coin depicts some (possibly zero) wealth on its face.
* A coin that depicts more wealth is more valuable, and two coins that
depict the same amount of wealth are equivalent.

Thus, a plain coin corresponds to 1, a coin depicting 2 plain coins is
worth omega^2, and in general, a coin depicting wealth x is worth omega^x.

-- Bachmann-Howard Ordinal --

Description:
* A coin can be made of silver or gold and depicts some (possibly zero)
wealth.
* Gold coins are more valuable than silver.
* For coins made of the same material, coins that depict more wealth are
more valuable (and two coins made of the same material and depicting the
same amount of wealth are equivalent).
* A silver coin may not depict a silver coin of greater value. Note:
Here 'depict' is transitive; a coin may depict another coin indirectly
through intermediate coin(s).

Bachmann-Howard ordinal Omega (or if we insert natural gaps in the
notation system, the least nonrecursive ordinal) corresponds to a plain
gold coin.  A gold coin depicting wealth x is worth omega^(Omega+x).  A
silver coin depicting wealth x is worth omega^(collapse of x) for an
appropriate ordinal collapsing function.  Also, because we have two
materials, coins can now depict more valuable coins, which necessitates
some restriction against infinite regress of coin values.

-- Pi^1_1-CA_0 --

By iterating the construction for Bachmann-Howard ordinal, we get a
notation system for Pi^1_1-CA_0 (same strength as KP + {there are n

Description:
* Coins are classified by the material (a natural number) and the amount
of wealth depicted.
* If m<n, then a coin made from material m is worth less than a coin
* For coins made from the same material, coins that depict more wealth
are worth more.
* A coin made from material n may not directly (as opposed to using
intermediate coin(s)) depict a coin of material above n+1.
(Equivalently, the total wealth depicted must be less than a plain coin
* A coin made from material n may not depict a more valuable coin made
from material n, except when it (the more valuable coin) is inside a
coin made from a material less than n.

-- Beyond --

Stronger ordinal notation systems can be made using just silver and gold
coins (gold coins more valuable than silver; coins from the same
material classified and ordered by the amount of wealth depicted), but
with more complicated rules about when a coin can depict a coin of
greater value.

-- Visualization --

While the description may be clear for FOM readers, nontechnical people
would benefit if there was a software system that allowed them to mint
and validate ordinal coins, see the coins on the screen, exchange coins,
and do other operations that will make the coins appear simple and
concrete.  For example, one useful operation is combining wealth by
pooling two sets of coins together; this correspond to symmetric ordinal
sum, which, unlike ordinary ordinal sum, is commutative.

Sincerely,
Dmytro Taranovsky
http://web.mit.edu/dmytro/www/main.htm
```