For questions on the formal system in mathematical logic for expressing computation using abstract notions of functions and combining them through binding and substitution.

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Lambda calculus: Simplifying booleans with beta reductions

I have been doing a homework assignment wherein I have been trying to determine the result of ((or true) false) using beta reduction. I began by writing the entire expression using lambda notation and ...
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22 views

How far can-I rewrite in lambda functions?

I am quite new with the lambda calculus. I am experimenting lambda-calculus proofs through the coq proof assistant, but the question I have is not related to coq (I guess). However, I'm going to use ...
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29 views

Map a set in mathematical notation

How would express the following JavaScript which takes a set and applies a lambda to each member of the set (resulting in a new set) in mathematical notation? ...
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Lambda calculus: composition of SKI

I am doing some exercises on writing a lambda term as a composition of the terms: S=$\lambda$xyz.xz(yz), K=$\lambda$xy.y, I=$\lambda$x.x. I know that all lambda terms can be written using S K and I ...
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Fixed points in computability and logic

I asked this question on CS.SE, too: http://cstheory.stackexchange.com/questions/27322/fixed-points-in-computability-and-logic I would like to understand better the relation between fixed point ...
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13 views

Union and intersection of Bohm trees

When I study the Bohm tree defined in The Lambda Calculus: Its Syntax and Semantics, H.P. Barendregt, Elseviser,$\cap\Phi$ or $\cup_i(M_i)$ always occurs. But I'm confused about the union and the ...
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45 views

Beta reduction: how to?

I'm trying to beta-reduce the following: $$\lambda xy.y((\lambda xyz.xyz)(\lambda u.u)(\lambda u.uu))$$ Anyway I think that I didn't understand terms' scope. Considering the application in the shape ...
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1answer
17 views

Why is this lambda calculus expression already in normal form?

I don't quite follow why the following expression is in normal form $$\lambda y.(y (\lambda z.w) (\lambda z.w))$$ I would have thought that reduces to $\lambda y.y w$, but according to ...
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123 views

Fixed point combinator and functions with no fixed point

In lambda calculus the fixed point combinator is defined as: It is very easy to see how $Yg =g(Yg)$ for any $g$ by using $\beta$-reduction. At the same time I wonder what is the meaning of ...
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1answer
30 views

Calculating $(\lambda x.(x+y))1$

$(\lambda x.(x+y))1$ returns(I think): $(1+y)$. Is $(1+y)$ another lambda function so I'd write: $\lambda y.(1+y)$ or is it just $(1+y)$ and if so what does that mean?
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In lambda calculus, why do we define $n=\lambda f.\lambda x.f^n(x)$ instead of $n=\lambda f.f^n$?

I just started learning lambda calculus and I understood most of it but i was thinking that why do we define $n=\lambda f.\lambda x.f^n(x)$ instead of $n=\lambda f.f^n$? I think it would be more ...
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49 views

In lambda calculus, can the parameter of abstraction be a non-variable lambda expression?

In short, my main confusion is between the two concept variable and lambda expression: I am reading this reference here: ...
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51 views

What is the desirable function identification when setting up arrows in the category of types?

My question is which functions can not be allowed in a statically typed programming language, so that the "canonical" category is less coarse than what you get if you define it's arrows to be ...
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2answers
59 views

In what sense is the S-combinator “substitution”?

According to the Wikipedia page on SKI-combinator calculus, I is the identity function, K is the constant function, and S is "substitution". I understand the first two, but I don't see what S has to ...
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2answers
104 views

Types versus kinds and sorts

In the context of logic, especially Higher‑Order‑Logic and Calculus‑of‑Construction, what is a kind and how does it relates to and differs from a type? My raw guess if that a kind is the higher level ...
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1answer
127 views

What does “calculus” mean?

"calculus" and "formal system" From http://en.wikipedia.org/wiki/Propositional_calculus#Terminology a calculus is a formal system that consists of a set of syntactic expressions ...
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55 views

Is this proof for the undecidability of $\beta$-normalisation in $\lambda$-calculus valid?

The proofs I have so far seen for the undecidability of $\beta$-normalisation all make use of Gödel numbering in order to first prove the more general Scott-Curry theorem. As an exercise, I have tried ...
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128 views

How to prove that these are the only type inhabitants?

Consider a type $$\mathsf{Boolean} = \forall \alpha.\ \alpha \to \alpha \to \alpha$$ with its two inhabitants \begin{align} \mathrm{tt} &= \lambda x.\ \lambda y.\ x \\ \mathrm{ff} &= \lambda ...
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33 views

Lambda calculus reducing expression

I have following expression to reduce: (λmnfx.mf(nfx) λfx.fx λzy.zzy) After some substitutions i get the result: ...
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47 views

Lambda calculus expression reduction

I don't know the correct answer how this reduction should've be done. Should I simply put λfx.fx in a place of m and λzy.zzy in a place of n? ...
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2answers
41 views

Understanding η-conversion (Lambda Calculus)

Let $h \in A\rightarrow (B\rightarrow C)$ I'm trying to understand the following reduction: $$\lambda x\in A. \lambda y \in B.(h(x))(y) \\= \lambda x\in A.h(x) \\= h$$ Apprantly, this is done by ...
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73 views

Extensional versus intensional theories in mathematics

Lambda calculus is often cited as an intensional theory whereas set theory is cited as an extensional theory. What are other examples of extensional and intensional theories of mathematical logic?
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80 views

Proving that $\Omega = (\lambda x.xx)(\lambda x.xx)$ is not typable in the simply typed lambda calculus

I am trying to prove that $\Omega = (\lambda x.xx)(\lambda x.xx)$ is not typable in the simply typed lambda calculus. Surprisingly, different textbooks and lecture notes do not contain that proof, ...
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30 views

Chaitin's constant for lambda calculus and combinatory logic

I have found some approximations of Chaitin's Constant for turing machines but I have not found approximations for others. I'd like to have a rough estimate or upper bound on it for lambda calculus ...
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1answer
86 views

What's the definition of equational theory? Why is λ logic free?

A book says that "λ is logic free: it is an equational theory." But I don't understand the "logic free" and "equational theory". Can you help me?
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1answer
40 views

Lambda calculus: encoding lists and projectors

Given a pair $[M_1,M_2]$ there is an easy encoding $\lambda x.x M_1 M_2$. For the n-tuple we have two options. First encoding: $$\lambda x.x M_1, M_2, \ldots , M_n$$ Second encoding: $$[M_0, [M_1 , ...
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69 views

Lambda Calculus using $\beta$-reductions

Use $\beta$ reductions to compute the final answer for the following $\lambda$ terms. Use a "fake" reduction step for "+" operator. Identify each redex for $\beta$-reduction steps. Does the order in ...
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1answer
164 views

Lambda calculus logical operators

Define the and operator in lambda calculus and prove your definition Define the exclusive or operator in lambda calculus, and prove your definition My answer for #1 is: AND $\equiv$ ...
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2answers
101 views

Using Combinators in Lambda Calculus

K $\equiv$ $\lambda$xy.x S $\equiv$ $\lambda$xyz.((xz)(yz)) Prove that the identify function I $\equiv$ $\lambda$x.x is equivalent to ((S K) K) I have no clue where to even start for ...
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1answer
38 views

The approximation rule implies the equality rule in systems of type assignments

I'm reading Barendregt's Lambda calculi with types (1992). In Proposition 4.1.4.1., he "proves" a lemma which shows the approximation rule implies the equality rule in typed lambda-calculi à la ...
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1answer
51 views

Clarification about the definition of term algebras

The following definition has been given in this article. A term algebra is an algebra $ \langle \mathcal{S}, \mathcal{G} \rangle $ where every time that $g_\alpha, g_\beta \in \mathcal{G}$ and $$ ...
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1answer
74 views

Why use *λx.x* instead of *f(x)*?

In my semantics class, we're learning about using (abusing?) lambda calculus. So far the professor hasn't imparted any reason for using λx.x instead of using f(x). Why use lambdas instead of basic ...
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117 views

How can I prove a simple eta-conversion?

I would like to prove the following: $$\lambda x.\ \lambda y.\ f\ z\ x\ y \overset{\eta}{=} \lambda x.\ f\ z\ x$$ Definitions Free variables $x \in FV(f) :\Leftrightarrow$ $x$ is a variable used ...
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1answer
89 views

Representing lists and trees in System F

System F (also known as second-order lambda calculus or polymorphic lambda calculus) is defined as follows. Types are defined starting from type variables $X, Y, Z, \ldots$ by means of two ...
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2answers
44 views

About the definition of fixed-point combinators

I am reading this wikipedia page to understand Fixed-point combinators: In computer science, a fixed-point combinator (or fixpoint combinator[1]) is a higher-order function y that satisfies the ...
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1answer
41 views

Beta reduction exercise question

I am trying to reduce the following $\lambda$-expression: $$(\lambda x.x x) (\lambda y.y x) z$$ So I am reducing to $$(\lambda y.y x) (\lambda y.y x) z$$ That reduces to $$(\lambda y.y x)xz$$ Now ...
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1answer
57 views

Pure Lambda Calculus: Call-by-value Free Variable Argument Application Reduction

In pure lambda calculus, under the call-by-value reduction strategy, a term of the form $(\lambda x. x)y \rightarrow y$ implies that the free variable $y$ is a value. However, only abstractions are ...
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Equivalence of categories of directed complete posets

In the book ``Domains and Lambda-Calculi'' by Amadio and Curien, there is the following exercise: Define an equivalence between the category of partial morphisms generated by $(\mathcal{M}_S, ...
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46 views

Proof of $(\forall x. \varepsilon(x)) \Rightarrow \bot $ in $\lambda\pi $ calculus $\equiv$

What is the right representation of the proof of $(\forall x. \varepsilon(x)) \Rightarrow \bot $ in simple type theory as a term of $\lambda\pi $ calculus $\equiv$? Note on notation: The epsilon ...
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1answer
84 views

Simple type theory: Proof inexistance of closed term

In simple type theory, how can I prove that there is no closed term of type? $$((P \Rightarrow Q) \Rightarrow Q) \Rightarrow P$$
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1answer
82 views

Consequences of difference between “strong” and weak Church-Rosser property

An Abstract rewriting system is a set A, whose elements are usually called objects, together with a binary relation on A, traditionally denoted by $\rightarrow$. An object $x \in A$ is ...
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35 views

Reductions under lambda in dependently typed lambda calculus

I am currently reading a Simon Thompson's Type Theory book. In chapter 5 he introduces a system TT(0,C), which limits a notion of reduction . In this notion of reduction, reductions under lambda are ...
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71 views

Is Lambda calculus a purely equational theory?

In a previous question I have been told that lambda calculs is pure syntax. I see that Lambda calculus is introduced inductively, but I don't see from what axioms it follows that: $$(\lambda x.x) M ...
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81 views

Meaning of variables and applications in lambda calculus

The wikipedia definition of lambda terms is: The following three rules give an inductive definition that can be applied to build all syntactically valid lambda terms: a variable, $x$, is ...
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2answers
56 views

Understanding second axiom of Primitive recursion

I read about Primitive recursion and was able to understand most of it. However I am finding it very difficult to understand the second axiom of primitive recursion. I can make out that it helps in ...
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Extracting values out of functions

Suppose that we add "typed" values into the untyped lambda calculus. Is it always possible to convert a lambda abstraction to it's equivalent typed value (like l y . 2 y becomes 2) without giving ...
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112 views

What's a good resource to learn about the simply typed lambda calculus?

I've read An Introduction to Functional Programming Through Lambda Calculus by Greg Michaelson, and found it to be a very good resource to learn about the untyped lambda calculus. However, I want to ...
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54 views

Is it possible to reduce a lambda expression to it's smallest equivalent form?

In the Untyped Lambda Calculus, is it possible to reduce any arbitrary expression to it's smallest equivalent form? (defined as an expression with the smallest number of lambda terms) If so, is there ...
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1answer
59 views

Lambda calculus - function reduction

I am trying to learn how to reduce functions in $\lambda$ calculus and I came across this task: Reduce this expression using normal strategy and applicative strategy. $(\lambda x.\lambda ...
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1answer
45 views

Formulate boolean logic in lambda calculus

I want to formulate the boolean operator $\Leftrightarrow$ in lambda calculus. I know that the negation is formulated as $\lambda x.x F T$ and the conjugation is formulated as $\lambda x y.x y F$ as ...