Additively indecomposable ordinal

In set theory, a branch of mathematics, an additively indecomposable ordinal α is any ordinal number that is not 0 such that for any $\beta,\gamma<\alpha$, we have $\beta+\gamma<\alpha.$ Additively indecomposable ordinals are also called gamma numbers.

From the continuity of addition in its right argument, we get that if $\beta < \alpha$ and α is additively indecomposable, then $\beta + \alpha = \alpha.$

Obviously 1 is additively indecomposable, since $0+0<1.$ No finite ordinal other than $1$ is additively indecomposable. Also, $\omega$ is additively indecomposable, since the sum of two finite ordinals is still finite. More generally, every infinite cardinal is additively indecomposable.

The class of additively indecomposable numbers is closed and unbounded. Its enumerating function is normal, given by $\omega^\alpha$.

The derivative of $\omega^\alpha$ (which enumerates its fixed points) is written $\epsilon_\alpha.$ Ordinals of this form (that is, fixed points of $\omega^\alpha$) are called epsilon numbers. The number $\epsilon_0=\omega^{\omega^{\omega^{\cdot^{\cdot^\cdot}}}}$ is therefore the first fixed point of the sequence $\omega,\omega^\omega\!,\omega^{\omega^\omega}\!\!,\ldots$

Multiplicatively indecomposable

A similar notion can be defined for multiplication. Besides 1, the multiplicatively indecomposable ordinals (also called delta numbers) are those of the form $\omega^{\omega^\alpha} \,$ for any ordinal α. Every epsilon number is multiplicatively indecomposable; and every multiplicatively indecomposable ordinal is additively indecomposable. The delta numbers are the same as the prime ordinals that are limits.

See also

• Ordinal arithmetic

References

• Sierpiński, Wacław (1958), Cardinal and ordinal numbers., Polska Akademia Nauk Monografie Matematyczne 34, Warsaw: Państwowe Wydawnictwo Naukowe, MR 0095787

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