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23
votes
Accepted
Why believe in the existence of large cardinals rather than just their consistency?
Without that existence, we wouldn't have any reason to believe in even the much weaker consistency assertions. … According to Steel, the large cardinal set-theorist can still consider the nature of V=L set theory, without the stronger large cardinals, simply by relativizing set theory to L. …
23
votes
3
answers
3k
views
Why believe in the existence of large cardinals rather than just their consistency?
What reasons are there to believe in their truth, rather than merely in their consistency, or at most their truth in some transitive model? … One might be $V = L$, or even the axiom of restriction, but most set theorists dismiss them, as they rule out large large cardinals and inaccessible cardinals, respectively, and instead seek core models …
13
votes
0
answers
1k
views
Is there any correspondence between Gödel and Kreisel that supports Kreisel's observation th...
National Acad., 1938, he remarks that V=L added as a new axiom seems to give a natural completion of the axioms of set theory, in so far as it determines the vague notion of an arbitrary infinite set in … So Davis (and apparently also Kreisel) believed that Gödel accepted V=L in 1938 as a new axiom. …
2
votes
0
answers
131
views
Generic behavior of the degree of a projective variety
Now, consider the incidence correspondence $X = \lbrace (v,L) \in V \times \mathrm{Gr}(k+1,n+1) \mid v \in L \rbrace \subset V \times \mathrm{Gr}(k+1,n+1)$. This is a projective variety. … \cap L) = 0$ for all $L \in U$. …
20
votes
Accepted
Are some interesting mathematical statements minimal?
Since it is a model of $\text{ZFC}+V=L$, one gets on board all of the consequences of that theory in the minimal model. … But there are many further properties true in the minimal model, when it exists, that are not provable from $\text{ZFC}+V=L$. …
14
votes
Why is inner model theory evidence for consistency of large cardinals?
I believe it is essentially this perspective that is expressed by the quotations you have made in the original question. … For example, if ZFC is consistent, then we know that it is consistent that ZFC + V=L + ¬Con(ZFC), with the point being that in any world fulfilling this theory, we would have the full fine-structural account …
12
votes
1
answer
512
views
Why do we need the comparison lemma?
way of building up the model $L[A]$), which have to have well-founded ultrapowers (in which case they are called mice), and one needs a comparison lemma for establishing which mice are initial segments … I read some of Steel’s “The Comparison Lemma” in my quest for an answer, and it mentioned that for $L$ the mice are just well-founded structures satisfying a fragment of $\mathrm{ZFC}$ and $V = L$, which …
1
vote
Standard models of N and R: An Alice/Bob approach
I don't believe "ultimate L" will ever gain popular traction, for reasons that should be apparent. … I think the question of whether $V$ equals $L$ is far more important to settle.
My opinion about $V=L$ is in the minority, but it has had some important supporters. …
4
votes
Standard models of N and R: An Alice/Bob approach
There cannot be a "true" $V$. There cannot even be a completed collection of all ordinals. … This rules out wanting $V=L$ as an axiom, as that limits the types of collections available. …
0
votes
What can one say about the Dirichlet problem for Schrödinger equation with negative potential?
I will assume $V \in L^\infty(\Omega)$ is smooth, $V>0$ and $g \in C^1(\partial \Omega)$. … {L^\infty}}{\lambda_1(\Omega)} \int_\Omega |\nabla u|^2 + 2\|V\|_{L^\infty} \int_\Omega w^2 \,, $$
hence
$$ J(u) \ge \left(1-\frac{2\|V\|_{L^\infty}}{\lambda_1(\Omega)} \right) \int_\Omega |\nabla u|^2 …
3
votes
A better way to explain forcing?
If we can add one then presumably we can add many, plus if we manage to add one then it already shows the independence of $V=L$ (by absoluteness of $L$ and the fact that $M[G]$ has the same ordinals), … I believe this shows that to some extent, Boolean-valued model does make forcing easier to understand. …
3
votes
Is $0^{(\omega)}$ a minimal cover in the arithmetic degrees
By arithmetical absoluteness, we have $$(\Phi_d^{g''})^{V[G,H]}=(\Phi_d^{g''})^{V[G]}$$ and so $r\in V[G]$; similarly, we get $r\in V[H]$. … On the other hand, if $\mathsf{V=L}$ then there is a $\Delta^1_2$ such $F$ (namely, "Pick the $L$-least upper bound"). …
2
votes
0
answers
117
views
What is the variance of a multi-type branching process?
{(l)}_{i{k_1}} M_{{k_1}k_2}\dots M_{k_{n-1}j} + M_{i{k_1}}V^{(l)}_{{k_1}{k_2}}M_{k_2k_3}\dots M_{k_{n-1}j}+\dots + M_{i{k_1}}M_{k_1k_2}\dots M_{k_{n-2}k_{n-1}}V^{(l)}_{k_{n-1}j}\right)\\
&=\left[V^{(l) … }M^{n-1}\right]_{ij}+\left[M V^{(l)}M^{n-2}\right]_{ij}+\left[M^2V^{(l)}M^{n-3}\right]_{ij}+\dots \left[M^{n-1}V^{(l)}\right]_{ij}\\
&=\sum_{k=0}^{n-1} \left[M^kV^{(l)}M^{n-1-k}\right]_{ij}
\end{align} …
18
votes
2
answers
1k
views
Has anyone seen this construction of the Weil representation of $\mathrm{Sp}_{2k}(\mathbb{F}...
The Heisenberg group comes with a natural action of $\Sp(V)$; I'll write it as $h \mapsto h^g$ for $h \in H$ and $g \in \Sp(V)$. … When $k=1$, I believe this is the full normalizer of $H$ in $\SL_p(\CC)$; when $k>1$, I think it is the normalizer of $H$ in $\SL_{p^k}(\QQ(\zeta))$. …
2
votes
1
answer
107
views
Representing solutions of $-\Delta u+au=f$ when $a\leq 0$
\end{cases} $$
It is easily seen that the variational formulation of the above equation is
$$(\nabla u,\nabla v)_{L^2}+(u,v)_{L^2}=(f,v)_{L^2},\;\;\;\;\text{ for all } v\in H^1_{per}(\Omega):=\{v\in H^ … (I believe it's some eigenspace). How to represent solution in this case? …