Recursion

The recursion is an important concept of googology. Basically, it meaning start from the some function, and then plugging back and diagonalizing it some number of times.

Recursive hierarchy of functions must have three rules: "brake" rule (starting function), "plugging" rule (it tells how many times the previous function must be plugged) and "decomposing" rule (for diagonalizing purpose, it finds the fundamental sequence of so-called limit ordinals).

Examples of recursive hierarchies are FGH, BEAF and Bird's one.