## Journal of Symbolic Logic

- J. Symbolic Logic
- Volume 54, Issue 2 (1989), 590-607.

### On Adding $(\xi)$ to Weak Equality in Combinatory Logic

Martin W. Bunder, J. Roger Hindley, and Jonathan P. Seldin

#### Abstract

Because the main difference between combinatory weak equality and $\lambda\beta$-equality is that the rule \begin{equation*}\tag{\xi} X = Y \vdash \lambda x.X = \lambda x.Y\end{equation*} is valid for the latter but not the former, it is easy to assume that another way of defining combinatory $\beta$-equality is to add rule $(\xi)$ to the postulates for weak equality. However, to make this true, one must choose the definition of combinatory abstraction in $(\xi)$ very carefully. If one tries to use one of the more common abstraction algorithms, the result will be an equality, $=_\xi$, that is either equivalent to $\beta\eta$-equality (and so strictly stronger than $\beta$-equality) or else strictly weaker than $\beta$-equality. This paper will study the relations $=_\xi$ for several commonly used abstraction algorithms, distinguish between them, and axiomatize them.

#### Article information

**Source**

J. Symbolic Logic, Volume 54, Issue 2 (1989), 590-607.

**Dates**

First available in Project Euclid: 6 July 2007

**Permanent link to this document**

https://projecteuclid.org/euclid.jsl/1183742929

**Mathematical Reviews number (MathSciNet)**

MR997891

**Zentralblatt MATH identifier**

0702.03007

**JSTOR**

links.jstor.org

#### Citation

Bunder, Martin W.; Hindley, J. Roger; Seldin, Jonathan P. On Adding $(\xi)$ to Weak Equality in Combinatory Logic. J. Symbolic Logic 54 (1989), no. 2, 590--607. https://projecteuclid.org/euclid.jsl/1183742929