 Solution

Interpret the language of ordered rings in the standard natural numbers, with nonlogical symbols $0,1,+,\times ,<$. We give a single first-order formula $F(n,y)$ defining $y=n!$, rather than a separate expression with $n$ multiplication signs. The key is Gödel beta-function sequence coding. The remainder relationuses only the allowed symbols; all variables range over $\mathbb{N}$. With $c>0$, it assigns a unique remainder $r$ at each position $i$.

The factorial graph formula via remainder coding asserts that one code contains the initial value $1$, the final value $y$, and every recurrence step. Here is the fully expanded formula, with no remainder-function or factorial symbol:The first equation represents remainder $1$ at position zero: its omitted explicit remainder bound follows from $c>0$. The second line represents remainder $y$ at position $n$. The universally checked equations represent the adjacent remainders and impose multiplication by $i+1$. Uniqueness of remainders therefore forces them, successively, to be $0!,1!,\dots ,n!$, proving $F(n,y)\Rightarrow y=n!$ by mathematical induction. When $n=0$, the recurrence clause is empty and the two endpoint clauses force $y=1$.

Conversely, choose a positive $c$ larger than every value in the finite sequence $0!,1!,\dots ,n!$ and divisible by each positive integer at most $n$. The moduli $d_i=1+(i+1)c$, $0\le i\le n$, are pairwise coprime. Indeed, for $i<j$, a common divisor of $d_i,d_j$ divides $(j-i)c$ and is coprime to $c$, so divides $j-i$. Since $j-i$ divides $c$, the common divisor must be one. The Chinese remainder theorem gives a nonnegative $b$ with $b\equiv i!(\mathrm{mod}{d}_i)$ for every $i$. The chosen values are smaller than their moduli, so they are the actual remainders; the appropriate quotient witnesses then make $F(n,n!)$ true. ThusThe formula has fixed length, independent of $n$. For example, writing its displayed syntax in plain text with one-character variables, ordinary parentheses, and explicit logical connectives takes fewer than $1000$ characters and fewer than $500$ logical-symbol tokens. Its length is therefore $O(1)$ as a uniform definition of the graph. Numeral names for particular input values, if substituted for the variables, add their own encoding lengths. Over the ordered ring $\mathbb{Z}$, restrict every quantifier to nonnegative integers and require $n,y\ge 0$; this gives the same definition on its nonnegative part. It is the chosen arithmetic structure, not ring axioms alone, that makes the formula define factorial.