The sphere eversion project

A Local sphere eversion

The local theory of Chapter 2 is already enough to deduce Smale’s sphere eversion theorem, although it is less natural than going through the general results of Chapter 3. The goal of this appendix is to explain how to do so. In this section \(E\) denote a finite dimensional real vector space equipped with an inner product. Later we will assume it is 3-dimensional. We denote by \(𝕊\) the unit sphere in \(E\).

Although we want to study immersions of \(𝕊\) into \(E\), we want to work only with functions defined on the whole \(E\). So we introduce a slightly artificial relation. We denote by \(B\) the open ball with radius \(9/10\) around the origin in \(E\) and set:

\[ \mathcal{R}:= \{ (x, y, φ) ∈ J^1(E, E) \; |\; x ∉ B ⇒ φ|_{x^⊥} \text{ is injective}\} . \]

Of course solutions of this relation restrict to immersions of \(𝕊\).

Lemma A.1
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The relation \(\mathcal{R}\) above is open.

Proof ▶

The main task is to fix \(x_0 \notin B\) and \(\varphi _0 \in L(E, E)\) which is injective on \(x_0^\perp \) and prove that, for every \(x\) close to \(x_0\) and \(\varphi \) close to \(\varphi _0\), \(\varphi \) is injective on \(x^\perp \). This is a typical situation where geometric intuition makes it feel like there is nothing to prove.

One difficulty is that the subspace \(x^\perp \) moves with \(x\). We reduce to a fixed subspace by considering the restriction to \(x_0^\perp \) of the orthogonal projection onto \(x^\perp \). One can check this is an isomorphism as long as \(x\) is not perpendicular to \(x_0\). More precisely, we consider \(f \! :J^1(E, E) \to ℝ \times L(x_0^\perp , E)\) which sends \((x, y, \varphi )\) to \((\langle x_0, x\rangle , \varphi \circ \operatorname{pr}_{x^\perp } \circ j_0)\) where \(j_0\) is the inclusion of \(x_0^\perp \) into \(E\). The set \(U\) of injective linear maps is open in \(L(x_0^\perp , E)\) and the map \(f\) is continuous hence the preimage of \(\{ 0\} ^c \times U\) is open. This is good enough for us because injectivity of \(\varphi \circ \operatorname{pr}_{x^\perp } \circ j_0\) implies injectivity of \(\varphi \) on the image of \(\operatorname{pr}_{x^\perp } \circ j_0\) which is \(x^\perp \) whenever \(\langle x_0, x\rangle \neq 0\).

Lemma A.2
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The relation \(\mathcal{R}\) above is ample.

Proof ▶

The core fact here is that if one fixes vector spaces \(F\) and \(F'\), a dual pair \((\pi , v)\) on \(F\) and an injective linear map \(\varphi \! :F \to F'\) then the updated map \(\Upsilon _{p}\left(\varphi , w\right)\) is injective if and only if \(w\) is not in \(\varphi (\ker \pi )\). First we assume \(\Upsilon _{p}\left(\varphi , \varphi (u)\right)\) is injective for some \(u\) in \(\varphi (\ker \pi )\) and derive a contradiction. We have \(\Upsilon _{p}\left(\varphi , \varphi (u)\right)v = \varphi (u)\) by the general definition of updating and also \(\Upsilon _{p}\left(\varphi , \varphi (u)\right)u = \varphi (u)\) since \(u\) is in \(\ker \pi \). Hence injectivity of \(\varphi \) ensure \(u = v\), which is absurd since \(\pi (u) = 0\) and \(\pi (v) = 1\). Conversely assume \(w\) is not in \(\varphi (\ker \pi )\) and let us prove \(\Upsilon _{p}\left(\varphi , w\right)\) is injective. Assume \(x\) is in the kernel of \(\Upsilon _{p}\left(\varphi , w\right)\). Decompose \(x = u + tv\) with \(u \in \ker \pi \) and \(t\) a real number. We have \(\Upsilon _{p}\left(\varphi , w\right)(x) = \varphi (u) + tw\). Hence our assumption on \(x\) implies \(t\) vanishes otherwise we would have \(w = -t⁻¹\varphi (u)\) contradicting that \(w\) isn’t in \(\varphi (\ker \pi )\). This vanishing and the assumption on \(x\) then imply \(\varphi (u) = 0\). Since \(\varphi \) is injective we conclude that \(u = 0\) and finally \(x = 0\).

We now turn to \(\mathcal{R}\). It suffices to prove that for every \(\sigma = (x, y, \varphi ) \in \mathcal{R}\) and every dual pair \(p = (\pi , v)\) on \(E\), the slice \(\mathcal{R}(\sigma , p)\) is ample. If \(x\) is in \(B\) then \(\mathcal{R}(\sigma , p)\) is the whole \(E\) which is obviously ample. So we assume \(x\) is not in \(B\). Since \(\sigma \) is in \(\mathcal{R}\), \(\varphi \) is injective on \(x^\perp \). The slice is the set of \(w\) such that \(\Upsilon _{p}\left(\varphi , w\right)\) is injective on \(x^\perp \). Assume first \(\ker \pi = x^\perp \). Then \(\Upsilon _{p}\left(\varphi , w\right)\) coincides with \(\varphi \) on \(x^\perp \) hence the slice is the whole \(E\). Assume now that \(\ker \pi \neq x^\perp \). The slice is not very easy to picture in this case. But one should remember that, up to affine isomorphism, the slice depends only on \(\ker \pi \). More specifically, if we keep \(\pi \) but change \(v\) then the slice is simply translated in \(E\). Here we replace \(v\) by the projection on \(x^\perp \) of the vector dual to \(\pi \) rescaled to keep the property \(\pi (v) = 1\). What has been gained is that we now have \(v \in x^\perp \) and \(x^\perp = (x^\perp \cap \ker \pi ) \oplus ℝ v\). Since \(\varphi \) is injective on \(x^\perp \), \(\varphi (x^\perp \cap \ker \pi )\) is a hyperplane in \(x^\perp \) and \(\Upsilon _{p}\left(\varphi , w\right)\) is injective on \(x^\perp \) if and only if \(w\) is in the complement of \(\varphi (x^\perp \cap \ker \pi )\) according to the core fact above. Since it is an hyperplane in \(x^\perp \), it has codimension at least \(2\) in \(E\) hence its complement is ample.

Theorem A.3 Smale 1958
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There is a homotopy of immersion of \(𝕊^2\) into \(ℝ^3\) from the inclusion map to the antipodal map \(a \! :q ↦ -q\).

Proof ▶

We denote by \(ι\) the inclusion of \(𝕊^2\) into \(ℝ^3\). We set \(j_t = (1-t)ι + ta\). This is a homotopy from \(ι\) to \(a\) (but not an immersion for \(t=1/2\)). Using the canonical trivialization of the tangent bundle of \(ℝ^3\), we can set, for \((q, v) ∈ T𝕊^2\), \(G_t(q, v) = \mathrm{Rot}_{Oq}^{πt}(v)\), the rotation around axis \(Oq\) with angle \(πt\). The family \(σ : t ↦ (j_t, G_t)\) is a homotopy of formal immersions relating \(j^1ι\) to \(j^1a\). Those formal solutions are holonomic when \(t\) is zero or one, so we can reparametrize the family to make such it is holonomic when \(t\) is close to zero or one. Then we can extend it to a homotopy of formal solutions of \(\mathcal{R}\) using a suitable cut-off ensuring smoothness near the orign. The relation \(\mathcal{R}\) is ample according to Lemma A.2 and then Lemma 3.21 ensures its 1-parameter version \(\mathcal{R}^ℝ\) is also ample. The relation \(\mathcal{R}\) is open according to Lemma A.1 hence \(\mathcal{R}^ℝ\) is also ample. So we can use Lemma 2.15 to deform our family of formal solutions into a holonomic one.