User:IssaRice/Computability and logic/Index and program

In computability theory, if we have some numbering ${\phi }_0,{\phi }_1,{\phi }_2,\dots$ of computable partial functions then the number $e$ is called the index of ${\phi }_e$, i.e., ${\phi }_e$ is the $e$th computable partial function.

We can also think of $e$ as the source code of an algorithm computing ${\phi }_e$.