Additively indecomposable ordinal
From the continuity of addition in its right argument, we get that if and α is additively indecomposable, then
is closed and unbounded, so the enumerating function of is normal. In fact,
The derivative (which enumerates fixed points of fH) is written Ordinals of this form (that is, fixed points of ) are called epsilon numbers. The number is therefore the first fixed point of the sequence
A similar notion can be defined for multiplication. The multiplicatively indecomposable ordinals are those of the form for any ordinal α.