# Combinators

*   Combinator Birds: Index of combinators with SKI equivalents

    [https://www.angelfire.com/tx4/cus/combinator/birds.html](https://www.angelfire.com/tx4/cus/combinator/birds.html)

Combinators are lambda abstractions without free variables, that is, closed lambda terms.

The **identity** function returns whatever argument it is applied to: $$\quad\ \lambda x.x$$

**Self application** function applies its argument to its argument:\
$$\quad\ \lambda f.ff$$
