# De Bruijn index

In mathematical logic, the De Bruijn index is a notation invented by the Dutch mathematician Nicolaas Govert de Bruijn for representing terms in the λ calculus with the purpose of eliminating the names of the variable from the notation.[1] Terms written using these indices are invariant with respect to α conversion, so the check for α-equivalence is the same as that for syntactic equality. Each De Bruijn index is a natural number that represents an occurrence of a variable in a λ-term, and denotes the number of binders that are in scope between that occurrence and its corresponding binder. The following are some examples:

• The term λx. λy. x, sometimes called the K combinator, is written as λ λ 2 with De Bruijn indices. The binder for the occurrence x is the second λ in scope.
• The term λx. λy. λz. x z (y z) (the S combinator), with De Bruijn indices, is λ λ λ 3 1 (2 1).
• The term λz. (λy. yx. x)) (λx. z x) is λ (λ 1 (λ 1)) (λ 2 1). See the following illustration, where the binders are coloured and the references are shown with arrows.

De Bruijn indices are commonly used in higher-order reasoning systems such as automated theorem provers and logic programming systems.[2]

## Formal definition

Formally, λ-terms (M, N, …) written using De Bruijn indices have the following syntax (parentheses allowed freely):

M, N, … ::= n | M N | λ M

where nnatural numbers greater than 0 — are the variables. A variable n is bound if it is in the scope of at least n binders (λ); otherwise it is free. The binding site for a variable n is the nth binder it is in the scope of, starting from the innermost binder.

The most primitive operation on λ-terms is substitution: replacing free variables in a term with other terms. In the β-reductionM) N, for example, we must:

1. find the variables n1, n2, …, nk in M that are bound by the λ in λ M,
2. decrease the free variables of M to match the removal of the outer λ-binder, and
3. replace n1, n2, …, nk with N, suitably increasing the free variables occurring in N each time, to match the number of λ-binders the corresponding variable occurs under when substituted.

To illustrate, consider the application

(λ λ 4 2 (λ 1 3)) (λ 5 1)

which might correspond to the following term written in the usual notation

x. λy. z xu. u x)) (λx. w x).

After step 1, we obtain the term λ 4 □ (λ 1 □), where the variables that are substituted for are replaced with boxes. Step 2 lowers the free variables, giving λ 3 □ (λ 1 □). Finally, in step 3 we replace the boxes with the argument; the first box is under one binder, so we replace it with λ 6 1 (which is λ 5 1 with the free variables increased by 1); the second is under two binders, so we replace it with λ 7 1. The final result is λ 3 (λ 6 1) (λ 1 (λ 7 1)).

Formally, a substitution is an unbounded list of term replacements for the free variables, written M1.M2…, where Mi is the replacement for the ith free variable. The increasing operation in step 3 is sometimes called shift and written ↑k where k is a natural number indicating the amount to increase the variables; For example, ↑0 is the identity substitution, leaving a term unchanged.

The application of a substitution s to a term M is written M[s]. The composition of two substitutions s1 and s2 is written s1 s2 and defined by

M [s1 s2] = (M [s1]) [s2].

The rules for application are as follows:

\begin{align} n [N_1\ldots N_n\ldots] =& N_n \\ (M_1\;M_2) [s] =& (M_1[s]) (M_2[s]) \\ (\lambda\;M) [s] =& \lambda\;(M [1.1[s'].2[s'].3[s']\ldots]) \\ & \text{where } s' = s \uparrow^1 \end{align}

The steps outlined for the β-reduction above are thus more concisely expressed as:

M) Nβ M [N.1.2.3…].

## Alternatives to De Bruijn indices

When using the standard "named" representation of λ-terms, where variables are treated as labels or strings, one has to explicitly handle α-conversion when defining any operation on the terms. The standard Variable Convention[3] of Barendregt is one such approach where α-conversion is applied as needed to ensure that:

1. bound variables are distinct from free variables, and
2. all binders bind variables not already in scope.

In practice this is cumbersome, inefficient, and often error-prone. It has therefore led to the search for different representations of such terms. On the other hand, the named representation of λ-terms is more pervasive and can be more immediately understandable by others because the variables can be given descriptive names. Thus, even if a system uses De Bruijn indices internally, it will present a user interface with names.

De Bruijn indices are not the only representation of λ-terms that obviates the problem of α-conversion. Among named representations, the nominal approaches of Pitts and Gabbay is one approach, where the representation of a λ-term is treated as an equivalence class of all terms rewritable to it using variable permutations.[4] This approach is taken by the Nominal Datatype Package of Isabelle/HOL.[5]

Another common alternative is an appeal to higher-order representations where the λ-binder is treated as a true function. In such representations, the issues of α-equivalence, substitution, etc. are identified with the same operations in a meta-logic.

When reasoning about the meta-theoretic properties of a deductive system in a proof assistant, it is sometimes desirable to limit oneself to first-order representations and to have the ability to (re)name assumptions. The locally nameless approach uses a mixed representation of variables—De Bruijn indices for bound variables and names for free variables—that is able to benefit from the α-canonical form of De Bruijn indexed terms when appropriate.[6][7]