
Under the summation it says $\tau=t$. So $\tau$ take the value of $t$ in the first Beta, $t+1$ in the 2nd beta, $t+2$ in the 3rd beta and so on (Is this right?)
If the above statement is right, first beta term will be $\beta^{t-t} = 0$, 2nd beta term will be $\beta^{t+1-t} = \beta^1$, and $\beta^2$ so on.
Following this logic, $\beta$ increases in this fashion, $\beta^0,\,\beta^1, \,\beta^2$, which is exactly the same thing you would get if you put $t=0$ under the summation sign and Beta^t as the first term in the summation.
What am I missing here?