 Solution

Fix an ordinal $\rho >1$. The Generalized Cantor normal form asserts that every nonzero ordinal $\alpha$ has a unique expressionProducts, sums and powers here are ordinal multiplication, ordinal addition and ordinal exponentiation; the coefficients need not be finite if $\rho$ is infinite. The case $\rho =\omega$ is Cantor normal form, and a finite base on finite ordinals gives ordinary positional notation. Zero has the empty expansion.

We first prove the ordinal division algorithm. For $\delta >0$, the function $q\mapsto \delta q$ is strictly increasing, continuous at limit ordinals and unbounded. Hence there is a greatest $q$ with $\delta q\le \alpha$: the first $q^{\prime }$ with $\delta q^{\prime }>\alpha$ cannot be a limit by continuity and must equal $q+1$. The interval after the initial segment $\delta q$ has a unique order type $r$, so $\alpha =\delta q+r$. Maximality of $q$ gives $r<\delta$. These facts also prove uniqueness of $q,r$.

Similarly $\beta \mapsto {\rho }^{\beta }$ is strictly increasing, continuous at limits and unbounded, so there is a unique largest ${\beta }_0$ with ${\rho }^{{\beta }_0}\le \alpha$. Divide by ${\rho }^{{\beta }_0}$ to obtainThe quotient is positive, and $\alpha <{\rho }^{{\beta }_0+1}={\rho }^{{\beta }_0}\rho$ makes ${\xi }_0<\rho$. If $r_0>0$, repeat on $r_0$; its leading exponent is strictly smaller than ${\beta }_0$. This process terminates, since an infinite run would give an infinite descending sequence of ordinals. It constructs the required form.

For uniqueness, a finite expression with largest exponent $\beta$ and coefficients below $\rho$ has value below ${\rho }^{\beta +1}$. Prove this from the end backwards: a tail with largest exponent $\gamma <\beta$ is below ${\rho }^{\gamma +1}\le {\rho }^{\beta }$, andThus the displayed leading exponent is the uniquely determined largest $\beta$ with ${\rho }^{\beta }\le \alpha$. The ordinal division algorithm uniquely determines its coefficient and the remaining tail, and recursion proves uniqueness of the whole expression. No commutative rearrangement of ordinal sums is used.