Hi

You may know that, if \(\displaystyle \beta>0,\) right multiplication ( \(\displaystyle y\mapsto\beta.y\) ) is strictly increasing. Therefore, the class of ordinals \(\displaystyle \{x\ ;\ \beta.x>\alpha\}\) is non empty and has a minimum, which must be a successor (or else, let's call this minimum \(\displaystyle \lambda,\) since right multiplication is continuous, we would have \(\displaystyle \beta.\lambda=\sup\{\beta.\delta\ ;\ \delta<\lambda\}\) and since \(\displaystyle \alpha<\beta.\lambda,\) this means there is a \(\displaystyle \delta<\lambda\) such that \(\displaystyle \alpha<\beta.\delta,\) absurd) so let's name this minimum \(\displaystyle \gamma+1\) and consider \(\displaystyle \gamma.\)

It is such that \(\displaystyle \beta.\gamma\leq\alpha\) and is maximal for this property.

Use now that right addition is also a strictly increasing continuous function, therefore there is a minimal ordinal \(\displaystyle x\) such that \(\displaystyle \beta.\gamma+x>\alpha,\) you find it again to be a successor, let's say \(\displaystyle x=\delta+1,\) and conclude. ( Also, if \(\displaystyle \delta\leq\gamma,\) since right addition is an (strictly) increasing function, you get \(\displaystyle \beta.\gamma+\delta\geq\beta.\gamma+\gamma=\beta.(\gamma+1)>\alpha\) by hypothesis on \(\displaystyle \gamma,\) contradiction; therefore \(\displaystyle \delta<\gamma\) )