Additively indecomposable ordinal explained
In set theory, a branch of mathematics, an additively indecomposable ordinal α is any ordinal number that is not 0 such that for any
, we have
Additively indecomposable ordinals were named the
gamma numbers by Cantor,
[1] p.20 and are also called
additive principal numbers. The
class of additively indecomposable ordinals may be denoted
, from the German "Hauptzahl".
[2] The additively indecomposable ordinals are precisely those ordinals of the form
for some ordinal
.
From the continuity of addition in its right argument, we get that if
and
α is additively indecomposable, then
Obviously 1 is additively indecomposable, since
No
finite ordinal other than
is additively indecomposable. Also,
is additively indecomposable, since the sum of two finite ordinals is still finite. More generally, every
infinite initial ordinal (an ordinal corresponding to a
cardinal number) is additively indecomposable.
The class of additively indecomposable numbers is closed and unbounded. Its enumerating function is normal, given by
.
The derivative of
(which enumerates its fixed points) 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 \omega,\omega\omega,\omega
,\ldots
Multiplicatively indecomposable
A similar notion can be defined for multiplication. If α is greater than the multiplicative identity, 1, and β < α and γ < α imply β·γ < α, then α is multiplicatively indecomposable. The finite ordinal 2 is multiplicatively indecomposable since 1·1 = 1 < 2. Besides 2, the multiplicatively indecomposable ordinals (named the delta numbers by Cantorp.20) are those of the form
for any ordinal
α. Every
epsilon number is multiplicatively indecomposable; and every multiplicatively indecomposable ordinal (other than 2) is additively indecomposable. The delta numbers (other than 2) are the same as the
prime ordinals that are limits.
Higher indecomposables
Exponentially indecomposable ordinals are equal to the epsilon numbers, tetrationally indecomposable ordinals are equal to the zeta numbers (fixed points of
), and so on. Therefore,
is the first ordinal which is
-indecomposable for all
, where
denotes
Knuth's up-arrow notation.
See also
Notes and References
- A. Rhea, "The Ordinals as a Consummate Abstraction of Number Systems" (2017), preprint.
- W. Pohlers, "A short course in ordinal analysis", pp. 27–78. Appearing in Aczel, Simmons, Proof Theory: A selection of papers from the Leeds Proof Theory Programme 1990 (1992). Cambridge University Press,