Paper 1, Section II, G

Logic and Set Theory
Part II, 2018

Give the inductive definition of ordinal exponentiation. Use it to show that αβαγ\alpha^{\beta} \leqslant \alpha^{\gamma} whenever βγ\beta \leqslant \gamma (for α1\alpha \geqslant 1 ), and also that αβ<αγ\alpha^{\beta}<\alpha^{\gamma} whenever β<γ(\beta<\gamma( for α2)\alpha \geqslant 2).

Give an example of ordinals α\alpha and β\beta with ω<α<β\omega<\alpha<\beta such that αω=βω\alpha^{\omega}=\beta^{\omega}.

Show that αβ+γ=αβαγ\alpha^{\beta+\gamma}=\alpha^{\beta} \alpha^{\gamma}, for any ordinals α,β,γ\alpha, \beta, \gamma, and give an example to show that we need not have (αβ)γ=αγβγ(\alpha \beta)^{\gamma}=\alpha^{\gamma} \beta^{\gamma}.

For which ordinals α\alpha do we have αω1ω1\alpha^{\omega_{1}} \geqslant \omega_{1} ? And for which do we have αω1ω2\alpha^{\omega_{1}} \geqslant \omega_{2} ? Justify your answers.

[You may assume any standard results not concerning ordinal exponentiation.]