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 , we have Additively indecomposable ordinals are also called gamma numbers or additive principal numbers. The class of additively indecomposable ordinals may be denoted , from the German "Hauptzahl". 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 Multiplicatively indecomposable A similar notion can be defined for multiplication. If α is greater than the