Example:
We toss a coin 3 times:
$\Omega$ = {$\omega_1 \omega_2 \omega_3$} = {HHH,HHT,HTH,TTT,THT,TTH,TTT}
2 times:
$\Omega$ = {$\omega_1 \omega_2$} = {HH,HT,TH,TT}
1 time:
$\Omega$ = {$\omega_1$} = {H,T}
What the heck does $\omega_1$ mean? I've read elsewhere that it's a singleton but really I don't know.
Does it mean:
{$\omega_1 \omega_2$} = {$\omega_1 *\omega_2$} = {H,T} * {H,T}
If we say that X is a stockprice that has on time $X_1$ three outcomes namely {Up(=U), Stays the same price(=EQ), Down(=D)} is
$\Omega$ = {$\omega_1$} = {U,EQ,D}
On time $X_2$
$\Omega$ = {$\omega_1 \omega_2$} = {U,EQ,D}*{U,EQ,D} = {UU,UEQ,UD,EQU,EQEQ,EQD,DU,DEQ,DDD}
I have looked at quite some answers like:
What is $\omega$ in probability theory?
Probability Notation: What does $\{\omega\in \Omega : X(\omega) \in A\}$ mean?
But still I don't understand. Can someone please give an example with real numbers like a stock price or the temperature with if not's too much work a graph?
Best Answer
Here $\omega_k$ is being used to represent "the result of the $k$th coin toss".
As JMoravitz commented, your source is taking the notation $\{\omega_1\omega_2\omega_3\}$ to represent the outcome set generated by the first three coin toss results. That is, as an abbreviation for $\{\langle\omega_1,\omega_2,\omega_3\rangle\in\{H,T\}^3\}$
So, yes, that does mean $\{\omega_1\omega_2\}$ is the cartesian product $\{\omega_1\}\times\{\omega_2\}$
So when instead using your stockmarket exsample, with the options $U,E,D$. (Advise: don't use two letters when one will do, especially when concatenating them into strings.)
$$\begin{align}\{\omega_1\omega_2\}&=\{U,E,D\}^2\\&=\{UU,UE,UD,EU,EE,ED,DU,DE,DD\}\end{align}$$