# Is $\omega_0-1$ infinite?

I have read in another answer Is infinity an odd or even number? that the $\omega_0$ is the "smallest infinity", but is $\omega_0-1$ not also infinite?

• People in this website are really mean. I wish I could tag it better, but I don't know how to. – user27221 Oct 23 '15 at 22:45
• $\omega_0$ is a limit ordinal, not a successor ordinal, so what do you mean by $\omega_0 - 1$? – Bungo Oct 23 '15 at 22:47
• Define $\omega_0-1$. If it's defined by the equation $1+x=\omega_0$ then it's equal to $\omega_0.$ The equation $x+1=\omega_0$ has no solution. – bof Oct 23 '15 at 22:52
• Correct, every ordinal is either a successor ordinal (i.e., is a successor of another ordinal, i.e. has a predecessor) or is not, in which case it is called a limit ordinal. In this case you have a limit ordinal. This is not to be confused with the fact that every ordinal has a successor, which is a consequence of well ordering. – Bungo Oct 23 '15 at 22:57
• There are ways to define $\omega_0 - 1$. For example, in the Surreal numbers. But even where it is defined, $\omega_0 - 1$ is still infinite. – Paul Sinclair Oct 24 '15 at 2:01

## 3 Answers

It is clear what is meant by $5-1$ or $\mbox{one billion} - 1$, but to ask a question about $\omega_{0} - 1$ you first need to ask, "what do I mean by that"? How does one subtract $1$ from an object like $\omega_{0}$, which isn't quite a number? I'm not quite sure how much you know about ordinals so I'll try to keep this self-contained.

There is a notion of $+1$ that makes sense even for objects like $\omega_{0}$. $\omega_{0}$ is the infinite collection of all the natural numbers, and $\omega_{0} + 1$ is the infinite collection of all the natural numbers together with $w_{0}$. Put another way, $\omega_{0} + 1$ is a collection that has all the natural numbers together with the symbol $\omega_{0}$. You can subtract $\omega_{0} + 1$ by $1$, and indeed $(\omega_{0} + 1) -1 = \omega_{0}$.

When you ask about $\omega_{0} - 1$ you are asking for some quantity, say $\kappa$, for which $\kappa + 1 = \omega_{0}$. That is, you are asking for some infinite collection such that, with the inclusion of one larger element, you obtain the collection of the natural numbers. However, every finite collection of natural numbers is contained in a larger finite collection. There is no infinite collection coming right before $\omega_{0}$. In set theoretic language, this means that $\omega_{0}$ is a limit ordinal. As a result, you cannot subtract one from it.

The kicker, here, is that you need to define $\omega_0-1,$ in the first place. Typically, given a successor ordinal $\alpha,$ one might say that $\alpha-1$ is the order type obtained by removing the greatest element from $\alpha.$ However, $\omega_0$ has no greatest element (since it isn't a successor ordinal), so we can't define $\omega_0-1$ in this way.

Another approach one might take is to say that $\alpha-1$ should be the order type of the set of all non-greatest elements of $\alpha$ (which is equivalent to the other way among successor ordinals). In that case, since all elements of $\omega_0$ are non-greatest elements, then we have in fact that $\omega_0-1=\omega_0,$ counterintuitive though that may be. This latter definition is generally undesirable, though, because while in the first definition, $0-1$ is undefined (as far as order types go), the latter definition would make $0-1=0,$ which is wildly undesirable.

Surely it's an infinte number.The fact is that you have exactly $\omega_0=\omega_0-1$. This can be proven knowing that $\omega_0$, the set of the natural numbers, is in bijection with $\omega_0-1$,that can be seen an the set of the naturals without the zero, via the function $n\mapsto n+1$,that is clearly bijective from $\omega_0$ to $\omega_0-1$. You can equally show this in the following way: for definition $\omega_0$ is the smallest of the infinte numbers.If $\omega_0-1$ was a finite number then also $\omega_0-1+1=\omega_0$ would be.Then $\omega_0-1$ is infinte and $\le\omega_0$.Since $\omega_0$is the smallest of the infinte numbers,it is necessary $\omega_0=\omega_0-1$.