Classification of totally real 3-dimensional - Project Euclid

J. Math. Soc.

Japan

Vol. 42, No. 4, 1990

Classification of totally real 3-dimensional submanifolds of with $K\geqq 1/16$ $S^{6}(1)$

By F. DILLEN, L. VERSTRAELEN and L.

VRANCKEN(*)

(Received Feb. 16, 1989) (Revised Sept. 13, 1989)

1. Introduction. does not admit any Kaehler It is well-known that a 6-dimensional sphere structure. However, using the Cayley algebra, a natural almost complex strucwhich itself is ture can be defined on considered as a hypersurface in viewed as the set of the purely imaginary Cayley numbers. And, together with the standard metric on , this almost complex structure determines a nearly Kaehler structure in the sense of A. Gray [G2]. In Section 2, we recall the , construction of this structure working with the 6-dimensional unit sphere (of radius and constant curvature 1). , two natural particWith respect to the almost complex structure on ular types of submanifolds $M$ can be investigated: those which are almost complex( . . for which the tangent space of $M$ at each point is invariant under the action of ) and those which are totally real ( . . for which the tangent space of $M$ at each point is mapped into the normal space at that point by ). are, as the The almost complex submanifolds of the nearly Kaehler invariant submanifolds of Kaehlerian manifolds, automatically minimal and even dimensional, and therefore of dimension 2 or 4. Moreover, A. Gray [G1] showed . So, that there do not exist 4-dimensional almost complex submanifolds in for this case, only the almost complex surfaces of need to be studied. Curvature properties for such surfaces were first obtained by K. Sekigawa [Se]. AS follows at once from their definition, for the other case, only 2- and 3. N. Ejiri [E1] proved dimensional totally real submanifolds can occur in is orientable and that every 3-dimensional totally real submanifold of minimal, and he first investigated curvature conditions on such manifolds. The were also considered, for in3-dimensional totally real submanifolds of stance, by H. Bl. Lawson Jr. and R. Harvey [H-L] in their study of calibrated geometries, and by K. Mashimo [M2] from the viewpoint of homogeneous manifolds. $S^{6}$

$R^{7}$

$S^{6}$

$J$

$g$

$S^{6}$

$J$

$S^{6}(1)$

$S^{6}(1)$

$J$

$i$

$e$

$i$

$J$

$e$

$J$

$S^{6}(1)$

$\Lambda f$

$S^{6}(1)$

$S^{6}(1)$

$S^{6}(1)$

$S^{6}(1)$

$S^{6}(1)$

(*)

Research Assistant of the Belgian National Science Foundation.

566

F. DILLEN, L. VERSTRAELEN and L. VRANCKEN

In our study of submanifolds of the nearly Kaehler 6-sphere, we concentrated on the following problems.

PROBLEM A. Which real numbers can be realized as the constant sectional curvatures of almost comPlex or minimal totally real submanifolds $M$ of $S^{6}(1)^{\rho}$

PROBLEM B. Let and be two consecutive numbers as in Problem A. Then, do there exist compact submanifolds $M$ of whose sectional curvatures $K$ satisfy , other than those for which or $K_{2}$

$K_{1}$

$S^{6}(1)$

$K\equiv K_{1}$

$K_{1}\leqq K\leqq K_{2}$

$K\equiv K_{2}^{p}$

In the more general situation, when $M$ is a minimal surface in a unit sphere $S^{n}(1)$ of arbitrary dimension , one has a complete answer to Problem A and partial answers to Problem B. Namely, O. Boruvka [Bo] constructed full $(i.e$ . not lying in a totally geodesic hypersurface of the ambient space) minimal immersions of 2-spheres $S^{2}(2/m(m+1))$ of constant Gauss curvature $K=2/m(m+1)$ into $S^{zm}(1)$ for every $m$ , Later, E. Calabi [Ca] showed that, up to rigid motions, these Boruvka spheres are the only compact minimal surfaces with constant Gauss curvature $>0$ in for any . Moreover, N. Wallach [Wa] proved that any minimal surface with constant Gauss curvature $K>0$ in $S^{n}(1)$ is locally an open subset of a Boruvka sphere, and, recently, R. Bryant [Br] proved that there are no minimal surfaces of constant negative Gauss curvature (whether, in this last statement, the condition on the negative in any sphere Gauss curvature to be constant can eventually be dropped, as far as we know, is still not settled [Y] . Concerning Problem , U. Simon [S-K] conjectured the following. $n$

$S^{n}(1)$

$n$

$S^{n}$

$)$

$B$

U. SIMON’S CONJECTURE. Let $M$ be a comPact surface which is minimally immersed in and whose Gauss curvature $K$ satisfies $2/m(m+1)\leqq K\leqq 2/m(m-1)$ . Then $K\equiv 2/m(m+1)$ or $K\equiv 2/m(m-1)$ , (and hence $M$ is for some a Boruvka sphere). $S^{n}(1)$

$m\in N\backslash \{0,1\}$

For $m=2$ and $m=3$ , this conjecture is known to be true, as was shown by H. Bl. Lawson [L], and by U. Simon and his coworkers [B-K-S-S], [K-S] essentially based on formulas for the Laplacian of certain functions of $K$ . Recently, quite a number of people have been working on this conjecture, using various methods and sometimes adding some additional assumption, such as T. Ogata, S. Montiel, T. Itoh, G. Jensen, M. Rigoli, J. Bolton, L. Woodward, and U. Simon, A. Schwenk and B. Opozda together with the present authors. As far as we know however, in general, for $m>3$ , this conjecture is still open. In our work in this field, yielding amongst others an alternative proof of this conjecture in case $m=2$ and $m=3$ (see for instance [D-V]), a crucial role is played by the method which is based on some integral formulas of A. Ros, whicb he first published in his solution [R] of a conjecture of K. Ogiue on

Totally real

submanifolds of

567

$S^{6}(1)$

K\"ahler submanifolds of complex projective spaces.

Proposition 3.1 of the present paper is obtained using this method. As the Lemma of H. Hopf, we believe that these integral formulas of A. Ros, which are given below, provide a powerful tool for the study of problems in global Riemannian geometry.

LEMMA OF A. ROS. Let $M$ be a comPact Riemannian manifold. Denote by UM the unit tangent bundle of $M$, and by $UM_{p}$ , the fiber of UM over a Point resPectively be the canonical measures on $M,$ UM of M. Let $dp$ , $du$ and and $UM_{p}$ . Then, for any continuous function $f:UMarrow R$ , one has $P$

$du_{p}$

$\int_{UM}fdu=\int_{M}(\int_{UM_{p}}fdu_{p})dp$

NOW, let

$T$

be any -covariant tensor $k$

fleld on M.

Then

$\int_{UM}(\nabla T)(u, u, \cdots u)du=0$

where

$\nabla$

is the Levi-Civita connection

A. Almost complex surfaces in Concerning Problem

$A$

,

of

$M$

.

,

.

$S^{6}(1)$

.

K. Sekigawa [Se] obtained the following.

THEOREM A. If an almost complex surface curvature $K$, then either $K=1$ (and $M$ is totally

has constant Gauss geodesic) or $K=1/6$ or $K=0$ .

in

$M$

$S^{6}(1)$

Moreover, for each of these possible cases, explicit examples are known (see, for instance, [Se]). The following results give a complete answer to Problem , for almost complex submanifolds. $B$

THEOREM B. Let Gauss curvature $K$. (a)

(b)

If If

$1/6\leqq K$

$M$

be a compact almost complex

(or equivalently

$0\leqq K\leqq 1/6$

,

then either

$1/6\leqq K\leqq 1$ $K\equiv 0$

or

),

surface in

then either

$K\equiv 1/6$

$K\equiv 1/6$

$S^{c}(1)$

or

zuhich

$K\equiv 1$

.

.

We obtained these results in [D-V-VI], and in [D-O-V-VI] together with B. Opozda, and our method of proof consisted in applying the Lemma of A. Ros constructed in terms of the second fundamental for some suitable tensors $M$ and its derivatives of van der Waerdenin form of the submanifold Bortolotti (see, for instance, [Ch]). We remark that (a) also follows from of $[0]$ . Theorem $T$

$S^{6}(1)$

$B$

F. DILLEN, L. VERSTRAELEN and L. VRANCKEN

568

B.

Totally real minimal surfaces in

$S^{6}(1)$

.

Whereas, as stated before, every 3-dimensional totally real submanifold of is minimal, in general this is not so in dimension 2. This can be seen can be isometrically for instance as follows. As we will mention later on, as a totally real and totally geodesic submanifold. Consider immersed in . Under the above immera small hypersphere $S^{2}(1/r^{2})$ of radius $r1/2$ . is chosen as an This is in contradiction with the fact that absolute maximum of . $e_{1}$

$f_{l}$

574

F. DILLEN, L. VERSTRAELEN and L. VRANCKEN

Case 4: Case 5:

$a\neq 0,$

$b=0,$ $c=0$

$a\neq 0,$

$b=0,$

we obtain a contradiction. $b=0,$ Case 6: $a\neq 0,$

.

$c\neq 0,$

Then, (4.25) immediately leads to a contradiction. $d=0$ . Applying the same argument as in Case 2,

. First, we deduce from (4.22) that $c^{2}=3/16$ . that $a=1/2$ . From (4.8), we then find that $d=1/2$ .

$c\neq 0,$

$d\neq 0$

Then, it follows from (4.21) NOW, putting $u=-1/\sqrt{5}\{e_{1}+e_{2}-\sqrt{3}e_{3}\}$ if $c=\sqrt{3}/4$ , and $u=-1/\sqrt{5}\{e_{1}+e_{2}+$ if $c=-\sqrt{3}/4$ , we see that $\langle h(u, u), Ju\rangle=\sqrt{5}/2>1/2$ . So, we obtain again a contradiction. is identically zero. By linearization, Case 7: $a=0$ . This implies that we then deduce that $b=c=d=0$ . In this case, all the equations are trivially $\sqrt{3}e_{3}\}$

$f_{1}$

satisfied. By combining this with Proposition 3.1, we immediately obtain the following lemma.

is a 3-dimensional compact totally real submanifold of and if all sectional curvatures $K$ of $M$ satisfy $K\geqq 1/16$ , then, for each point $M$ , there exists an orthonomal basis of $T_{p}M$ such that either

LEMMA 4.1.

If

$M$

$\{e_{1}, e_{2}, e_{3}\}$

(i)

(4.28)

$h(e_{1}, e_{1})=h(e_{2}, e_{2})=h(e_{3}, e_{3})=0$

,

$h(e_{1}, e_{2})=h(e_{1}, e_{3})=h(e_{2}, e_{3})=0$

,

$or$

(ii)

(4.29)

$h(e_{1}, e_{1})= \frac{\sqrt{5}}{2}Je_{1}$

,

$h(e_{2}, e_{2})=- \frac{\sqrt{5}}{4}Je_{1}+\frac{\sqrt{10}}{4}Je_{2}$

$h(e_{3}, e_{3})=- \frac{\wedge 5}{4}Je_{1}-\frac{\sqrt{10}}{4}Je_{2}$

$h(e_{1}, e_{3})=- \frac{\sqrt{5}}{4}Je_{3}$

,

,

,

$h(e_{1}, e_{2})=- \frac{\wedge 5}{4}Je_{2}$

$h(e_{2}, e_{3})=- \frac{\sqrt{10}}{4}Je_{3}$

,

,

$or$

(4.30)

Let

(iii)

$M$

$h(e_{1}, e_{1})= \frac{\sqrt{5}}{2}Je_{1}$

,

$h(e_{2}, e_{2})=- \frac{\sqrt{5}}{4}Je_{1}$

,

$h(e_{3}, e_{3})=- \frac{\sqrt{5}}{4}Je_{1}$

,

$h(e_{1}, e_{2})=- \frac{\sqrt{5}}{4}Je_{2}$

$h(e_{1}, e_{3})=- \frac{\sqrt{5}}{4}Je_{3}$

,

$h(e_{2}, e_{3})=0$

be as in Lemma 4.1.

,

.

Then we have the following proposition.

PROPOSITION 4.1. Let $p\in M$ . Then we have that, (a) if (4.28) holds, then $K(p)\equiv 1$ ; (b) if (4.29) holds, then $K(p)\equiv 1/16$ ; (c) if (4.30) holds, then $1/16\leqq K(p)\leqq 21/16$ ,

$S^{6}(1)$

$p$

of

Totally real

submanifolds of

575

$S^{6}(1)$

where 1/16 and 21/16 are actually obtained.

is a geodesic point. From the Gauss PROOF. (a) In this case, $h=0$ , so equation, we obtain that $K(p)\equiv 1$ . (b) From [E1], we find that has the same form as the second fundamental form of a totally real submanifold of constant curvature 1/16. So $K(p)$ $=1/16$ by the Gauss equation. (c) From the Gauss equation and (4.30), we obtain that $P$

$h_{p}$

$R(e_{1}, e_{2})e_{2}=R(e_{1}, e_{3})e_{3}= \frac{1}{16}e_{1}$

,

$R(e_{2}, e_{3})e_{3}= \frac{21}{16}e_{2}$

$R(e_{1}, e_{2})e_{3}=R(e_{2}, e_{3})e_{1}=R(e_{3}, e_{1})e_{2}=0$

Let basis {X,

,

.

be any plane section of $T_{p}M$ . Then we can find an orthonormal $Y$ and $Y=\sin\varphi e_{1}-\cos\varphi\sin\theta e_{2}+$ such that $X=\cos\theta e_{2}$ -Fsin } of , where . Then,

$\sigma$

$\theta e_{3}$

$\sigma$

$\cos\varphi\cos\theta e_{3}$

$\theta,$

$\varphi\in R$

$R(X, Y, Y, X)=\cos^{2}\theta R(e_{2}, Y, Y, e_{2})+2\cos\theta\sin\theta R(e_{2}, Y, Y, e_{3})$ $+\sin^{2}\theta R(e_{3}, Y, Y, e_{3})$

$=\cos^{2}\theta\sin^{2}\varphi R(e_{1}, e_{2}, e_{2}, e_{1})+\cos^{2}\varphi R(e_{2}, e_{3}, e_{3}, e_{2})$

$+\sin^{2}\theta\sin^{2}\varphi R(e_{1}, e_{3}, e_{3}, e_{1})$

$= \frac{1}{16}\perp\frac{20}{16}\cos^{2}\varphi$

,

and so we have $K( \sigma)=\frac{1}{16}+\frac{20}{16}\cos^{2}\varphi$

,

which gives us $1/16\leqq K\leqq 21/16$ , where 1/16 is attained when $\cos\varphi=0,$ $i.e$ . when passes through , and 21/16 is attained only when $\cos\varphi=\pm 1$ , i.e. the plane by the plane spanned by and . $\sigma$

$e_{1}$

$e_{2}$

$\blacksquare$

$e_{3}$

The next statements follow easily from Proposition 4.1.

COROLLARY 4.1. If $M$ is a 3-dimensional compact totally real submanifold and if the sectional curvatures $K$ of $M$ satisfy either $1/16\leqq K\leqq 1$ or 1/16 of $\leqq K