# Thread: Hilbert's axioms. Betweenness and ordered fields.

1. ## Hilbert's axioms. Betweenness and ordered fields.

I am reading the book "Geometry: Euclid and Beyond - Robin Hartshorne". Here's the first half of Proposition 15.3 from the book.

If $F$ is a field, and if there is a notion of betweenness in the Cartesian plane $\Pi_F$ satisfying Hilbert's axioms (B1)-(B4), then $F$ must be an ordered field.

The proof in the book reads as follows:

Suppose that $F$is a field and there is a notion of betweenness in the plane $\Pi_F$ satisfying (B1)-(B4). We define the subset $P \subset F$ to consist of all $a \in F$ such that the point $(a,0)$ of the x-axis is on the same side of $0$ as $1$.
"Now one can easily show that $a,b \in P \Rightarrow a+b \in P$."

... Which I am not able to show and so I need your help.

I was able to prove, using Pasch's axiom of betweenness, a.k.a (B4) in the book, that $(0,0)*(1,0)*(a,0) \Rightarrow (0,0)*(0,1)*(0,a)$ .But now I am stuck. Can someone help.

2. ## Re: Hilbert's axioms. Betweenness and ordered fields.

Originally Posted by abhishekkgp
I am reading the book "Geometry: Euclid and Beyond - Robin Hartshorne". Here's the first half of Proposition 15.3 from the book.
If $F$ is a field, and if there is a notion of betweenness in the Cartesian plane $\Pi_F$ satisfying Hilbert's axioms (B1)-(B4), then $F$ must be an ordered field.
The proof in the book reads as follows:
Suppose that $F$is a field and there is a notion of betweenness in the plane $\Pi_F$ satisfying (B1)-(B4). We define the subset $P \subset F$ to consist of all $a \in F$ such that the point $(a,0)$ of the x-axis is on the same side of $0$ as $1$.
"Now one can easily show that $a,b \in P \Rightarrow a+b \in P$."
... Which I am not able to show and so I need your help.
I was able to prove, using Pasch's axiom of betweenness, a.k.a (B4) in the book, that $(0,0)*(1,0)*(a,0) \Rightarrow (0,0)*(0,1)*(0,a)$ .But now I am stuck. Can someone help.
I think you are going have to list Hartshorne's version of Hilbert' (B1)-(B4),
Also how is a+b defined?

3. ## Re: Hilbert's axioms. Betweenness and ordered fields.

Originally Posted by Plato
I think you are going have to list Hartshorne's version of Hilbert' (B1)-(B4),
Also how is a+b defined?
B1) If B is between A and C, (written A*B*C), then A, B, C are three distinct points on a line, and also C*B*A.
B2) For any two distinct points A, B, there exists a point C such that A*B*C
B3) Given three distinct points on a line, one and only one of them is between the other two.
B4) Let A, B, C be three non-collinear points, and let $l$ be a line not containing any of A,B,C. If $l$ contains a point D lying between A and B, then it must also contain either point lying between A and C or a point lying between B and C, but not both.

As for $a+b$, the '+' here is the field addition and $a,b \in F$.

4. ## Re: Hilbert's axioms. Betweenness and ordered fields.

Originally Posted by abhishekkgp
B1) If B is between A and C, (written A*B*C), then A, B, C are three distinct points on a line, and also C*B*A.
B2) For any two distinct points A, B, there exists a point C such that A*B*C
B3) Given three distinct points on a line, one and only one of them is between the other two.
B4) Let A, B, C be three non-collinear points, and let $l$ be a line not containing any of A,B,C. If $l$ contains a point D lying between A and B, then it must also contain either point lying between A and C or a point lying between B and C, but not both.
As for $a+b$, the '+' here is the field addition and $a,b \in F$.
Well you have succeeded in confusing me. I taught the axiomatic geometry many times and hence Hilbert's betweenness axioms. I have never seen them applied to the set of positives $,\mathcal{P},$ in an ordered field.

From your OP, I assumed that the set $\mathcal{P}$ was defined a half line in the x-axis. In which case, it is a convex set and the result should follow. I asked about $+$ operation because it seems to me that you are required to show that $(a+b.0)$ is in the 1-side of zero.

Sorry, but I simply do not understand you notation.

5. ## Re: Hilbert's axioms. Betweenness and ordered fields.

Originally Posted by Plato
Well you have succeeded in confusing me. I taught the axiomatic geometry many times and hence Hilbert's betweenness axioms. I have never seen them applied to the set of positives $,\mathcal{P},$ in an ordered field.

From your OP, I assumed that the set $\mathcal{P}$ was defined a half line in the x-axis. In which case, it is a convex set and the result should follow. I asked about $+$ operation because it seems to me that you are required to show that $(a+b.0)$ is in the 1-side of zero.

Sorry, but I simply do not understand you notation.
Does the following make sense?

If $F$ is a field, and if there is a notion of betweenness in the Cartesian plane $\Pi_F$ satisfying Hilbert's axioms (B1)-(B4), then $F$ must be an ordered field.
Here $\Pi_F$ is the cartesian plane on the field $F$, that is, the set of all ordered pairs $(x,y)$, where $x,y \in F$.