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Jun
25
awarded  Notable Question
May
22
awarded  Civic Duty
May
21
awarded  Yearling
May
16
comment To prove a language is not recursive
Suppose you want to find out whether some Turing machine accepts at least one word, so you run it in parallel on more and more inputs waiting to see if it would halt at least on one of them. Can you see the analogy now? Its formalisation is the solution.
May
11
comment The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
The true comment above relates to the original question where I did not include the assumption of $GCH$.
May
11
revised The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
added 4 characters in body
May
11
revised The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
added 51 characters in body
May
11
revised The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
Attempt to give more context to the question.
May
8
accepted The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
May
8
revised The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
added 68 characters in body
May
8
comment The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
I forgot to mention I work in Godel's constructible $L$ and hence $2^\lambda = \lambda^+ = \kappa$.
May
8
asked The number of subsets of cardinality less than $\kappa$ of a cardinal $\kappa$ is $\kappa$
Apr
20
comment Opposite category functor
But sperners lemma's answer above gave an isomorphism $Dual : \mathcal{C} \to \mathcal{C}^{op}$, so $\mathcal{C}$ must be equivalent to $\mathcal{C}^{op}$.
Apr
18
accepted What does it mean for pullbacks to preserve monomorphisms?
Apr
16
asked What does it mean for pullbacks to preserve monomorphisms?
Mar
11
asked What is the least ordinal $\beta$ for which the function $f_\beta(n)$ in fast-growing hierarchy is incomputable?
Mar
8
comment The rationale behind the oracle machine notation with brackets $\{e\}^A$
@QuinnCulver Bob's reply was exactly the one in the quotes, whatever the origin may be (even if he knew), clearly he did not consider it worth the time explaining it.
Mar
8
accepted Errata for Rogers' Computability book
Mar
8
accepted What is an omega model?
Mar
8
comment The rationale behind the oracle machine notation with brackets $\{e\}^A$
@QuinnCulver I asked, confer the answer.