Remarkable cardinal explained

In mathematics, a remarkable cardinal is a certain kind of large cardinal number.

A cardinal κ is called remarkable if for all regular cardinals θ > κ, there exist π, M, λ, σ, N and ρ such that

1. π : M → H_θ is an elementary embedding
2. M is countable and transitive
3. π(λ) = κ
4. σ : M → N is an elementary embedding with critical point λ
5. N is countable and transitive
6. ρ = M ∩ Ord is a regular cardinal in N
7. σ(λ) > ρ
8. M = H_ρ^N, i.e., M ∈ N and N ⊨ "M is the set of all sets that are hereditarily smaller than ρ"

Equivalently,

\kappa is remarkable if and only if for every λ >\kappa there is \bar λ <\kappa such that in some forcing extension V[G] , there is an elementary embedding j:V ^{V →} _{\bar λ} V ^{V} _{λ} satisfying j(\operatorname{crit}(j)) = \kappa . Although the definition is similar to one of the definitions of supercompact cardinals, the elementary embedding here only has to exist in V[G] , not in V .

See also

• Hereditarily countable set