Paper 1, Section II, F

Logic and Set Theory
Part II, 2016

Which of the following statements are true? Justify your answers. (a) Every ordinal is of the form α+n\alpha+n, where α\alpha is a limit ordinal and nωn \in \omega. (b) Every ordinal is of the form ωαm+n\omega^{\alpha} \cdot m+n, where α\alpha is an ordinal and m,nωm, n \in \omega. (c) If α=ωα\alpha=\omega \cdot \alpha, then α=ωωβ\alpha=\omega^{\omega} \cdot \beta for some β\beta. (d) If α=ωα\alpha=\omega^{\alpha}, then α\alpha is uncountable. (e) If α>1\alpha>1 and α=αω\alpha=\alpha^{\omega}, then α\alpha is uncountable.

[Standard laws of ordinal arithmetic may be assumed, but if you use the Division Algorithm you should prove it.]