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Jun
16
comment Are there collections of sets that are neither a set nor a definable proper class?
I'd be interested to see some examples. But for now I think Asaf's answer is probably what you want.
Jun
16
comment Are there collections of sets that are neither a set nor a definable proper class?
You can only really formalize questions about proper classes in ZFC when they are definable proper classes. (The way that you formalize these questions is by using the definition of the definable proper class!) So in a sense, the answer to your question is "no".
May
7
comment what does function from a set to its power set mean?
@Memming: the difference is probably that rahul got tired of writing dashes.
May
7
comment what does function from a set to its power set mean?
I don't understand the downvote. Isn't this exactly the kind of question we want to answer in the elementary set theory tag?
May
7
comment what does function from a set to its power set mean?
Just define $f(1) = \{2,3\}, f(2) = \{1,2,3\}, f(3) = \{3\}$. The point is that functions don't have to be defined from some "rule"; they can be defined in an almost completely arbitrary fashion. If you pick any three elements of $P(A)$ (even allowing repetitions!) you can make a function which maps to those elements.
May
7
answered what does function from a set to its power set mean?
Apr
30
comment Set Theory Proof: Valid or not?
I don't understand your notation in 1, 2, and 3. Usually set theorists (and mathematicians in general) use { ... | ... } to mean a particular set, which is like a mathematical object; in a proof, you need to be writing statements (about mathematical objects).
Apr
27
comment The genus of a certain kind of cubic
And a singularity is a double point or cusp?
Apr
27
comment The genus of a certain kind of cubic
So, if $a_3$ is nonzero in my question, then the curve is not elliptic and hence has genus $0$?
Apr
27
comment The genus of a certain kind of cubic
Are you saying that for the curve to have positive genus it must be of the form $y^2 = x^3 + ax + b$, with $a$ and $b$ rational? Or is "genus $1$" the definition of "elliptic"?
Apr
26
asked The genus of a certain kind of cubic
Apr
17
asked Does there exist a quadratic generalization of the continued fraction approximants?
Mar
10
comment Jech's Set Theory logic prerequisites
@Nectric: I'd say Jech's book is actually fairly self-contained; but its scope is far too large for it to be anything other than a reference. Plus, its treatment of certain topics (forcing in particular) is not what I would want for a beginner.
Mar
10
answered Jech's Set Theory logic prerequisites
Mar
6
answered Existence of an uncountable set of sequence
Feb
24
revised Initial segments of trees
Not all subsets are initial segments.
Feb
21
awarded  Enthusiast
Feb
18
comment Questions of $\mathbb{P}$-name for a set and functions
Which $p$ are you talking about in your two questions?
Feb
17
answered Initial segments of trees
Feb
13
comment Question of $\Diamond$ in Generic Extension
Apply $\diamondsuit$ to the $\mathbb{P}$-names for subsets of $\omega_1$? (By the ccc, you can code such a name as an $\omega_1$-sequence of countable subsets of $\mathbb{P}$.)