BIG FOOT is a counterpart of Rayo's number based on an extended version of the language of first-order set theory. As a result, it is among the largest named numbers. It was defined in October 2014 by an author under the pen name "Wojowu" or "LittlePeng9", and was given its name by Sbiis Saibian.[1]

Its definition is almost identical to Rayo's number, another well-known large number which diagonalizes over first-order formulas in the von Neumann universe (which is the universe of discourse for first-order set theory). BIG FOOT extends first-order set theory by making use of a unique domain of discourse called the oodleverse, using a language called first-order oodle theory (FOOT), and generalizing nth-order set theory of arbitrarily large n.

Letting \(\text{FOOT}(n)\) denote the largest natural number uniquely definable in the language of FOOT in at most \(n\) symbols, we define BIG FOOT as \(\text{FOOT}^{10}(10^{100})\), where \(\text{FOOT}^{a}(n)\) is \(\text{FOOT}(n)\) iterated \(a\) times (recursion). BIG FOOT is thus equal to:


Definition of FOOT

The language of first-order oodle theory is defined as the language of set theory augmented with the symbols \([\) and \(]\). The universe of discourse consists of oodles, which are subject to the Tarskian definition of truth for a set theory. We call \(\in\)-transitive oodles oodinals, and consider \(\in\) as the ordering relation amongst them (so that we can speak of "larger" and "smaller" oodinals).

The FOOT function is the oodle-theory analogue to Rayo's function, where oodle-theory is an extension of set-theory.

Because all the structures involved are elements of the universe of discourse, FOOT has turned out to be equivalent in strength to FOST with a single truth predicate adjoined.[2]


  1. Wojowu and Nathan Ho. First-order oodle Retrieved 2014-11-11. [dead link]
  2. FOOT is not as strong as I thought, LittlePeng9's user blog

See also