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A smooth 4-manifold is a 4-manifold with a smooth structure. In dimension four, in marked contrast with lower dimensions, topological and smooth manifolds are quite different.

2 votes

Knotted projective planes and fake complex projective space

I can't think of another way to get $CP^2$ from an embedded $RP^2$ in the 4-sphere, so I would guess you are right. But why would the knottedness of the $RP^2$ imply that the $CP^2$ is exotic? After …
Danny Ruberman's user avatar
5 votes

Open book decompositions in dimension 4

Nice question; there's not much known in general. The signature obstruction you mention goes back to Winkelnkemper (Bull. Amer. Math. Soc. 79 (1973), 45–51) and is sufficient for simply connected $4n$ …
Danny Ruberman's user avatar
12 votes
Accepted

Behavior of genus function on a 4-manifold for sums

In the case that $x\cdot x \neq 0$, topological methods based on the G-signature show that the genus goes to infinity more or less quadratically in $n$. (I'll be more specific below.) This goes back t …
Danny Ruberman's user avatar
1 vote

Approximation theorem for Anti-Self-Dual Metrics

There are some results in this general direction, but on non-compact manifolds. See for instance Instanton approximation, periodic ASD connections, and mean dimension, by Shinichiroh Matsuo, Masaki T …
Danny Ruberman's user avatar
7 votes
Accepted

Distinguishing homology $S^1 \times S^2$'s which bound homotopy $S^1$'s

A standard construction would be to take a 3-manifold $Y$ given by 0-surgery on a knot $K$. If $K$ is the boundary of a slice disk $D \subset B^4$, then the complement, say $W$ of a neighborhood of $D …
Danny Ruberman's user avatar
3 votes
Accepted

$\pi_1$ of 4-manifolds that "look like" disk bundles

Let's take your $p$ to be prime. Then $X$ has to be simply connected, even without all of the hypotheses. Here is the argument. From the map on $\pi_1(L) \to Z_p$ you get a map $L \to BZ_p$. This ma …
Danny Ruberman's user avatar
17 votes
Accepted

Handlebody decomposition of an open 4-manifold

There are not that many explicit handlebody pictures of exotic open 4-manifolds, because they get awfully complex in short order. The ones that I know of are in work of Žarko Bižaca from the mid-90's …
Danny Ruberman's user avatar
7 votes
Accepted

Embedding problem for 3-manifolds attacked via 4-manifolds

Since you've asked for an opinion, my answer is also an opinion. I would say that the embedding in $R^5$ is not going to be helpful. You are seeking to take $M = \partial N_i$ and `improve' the $N_i$ …
Danny Ruberman's user avatar
15 votes
Accepted

Characteristic class that cannot be represented by disjoint tori

In $H_2(CP^2)$, every class $nH$ where $H$ is a generator and n is odd is characteristic. However, if $n >3$, then such a class is not represented by a torus. It is not represented by a disjoint unio …
Danny Ruberman's user avatar
7 votes
Accepted

Invertible 2-knots in $S^4$

Q1: This is true in the topological category and unknown in the smooth setting. In the topological setting, the fundamental group of $S^4 - K_1 \# K_2$ is $G_1 *_\mathbb{Z} G_2$ where $G_i$ are the fu …
Danny Ruberman's user avatar
9 votes

Double ($p$-fold) coverings of $B^4$ along ribbon/slice disks

The first is in Akbulut-Kirby, Branched covers of surfaces in 4-manifolds. (Math. Ann. 252, 111-131 (1980). See the end of Section 3; it's basically explained via a single example (the square knot rib …
Danny Ruberman's user avatar
9 votes
Accepted

$0$-surgery of slice knots and contractible manifolds

Yes, this can be done, but requires a little care with the fundamental group. First, let me tighten up your description; one is attaching the 2-handle to $S^1 \times B^3$ along a curve $\gamma$ in its …
Danny Ruberman's user avatar
6 votes

Kirby diagrams of Mazur manifolds

The paper of Fickle (not the paper of Gordon to which you link) has an explicit construction for $\Sigma(3,5,19)$. It's behind a paywall but maybe you can get it from interlibrary loan or some kind so …
Danny Ruberman's user avatar
3 votes

4-genus of a 2-bridge link

(This is really a comment on the answer relating to concordance order.) Since your p is even, then your 2-bridge knot is actually a link. So, while it makes sense to ask if it's a slice or ribbon l …
Danny Ruberman's user avatar
7 votes
Accepted

Akbulut's cork involution

The boundary of W may be described as 0-framed surgery on both components of the link you drew. The link can be drawn in a more symmetric fashion, so that it is clear that there is an involution inter …
Danny Ruberman's user avatar

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