$<\backslash$Sigma

$<\backslash$Sigma

$<\backslash$Sigma

$<\backslash$Sigma

$<(1)$

If the sequence $(1)$ contains only a finite number of distinct $\backslash$Sigma $$

$...$

$$

$,$ then

at least two of the $\backslash$Sigma $$

$...$

$$

in $(1)$ are the same stage. Use this conclusion

and properties of $<$ to get a contradiction