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Let $\M,\N$ be $d$-dimensional oriented Riemannian manifolds. Consider for a moment the Dirichlet functional $E:C^{\infty}(M,N) \to \R$, give by
$$E(f)=\int_{\M} |  df|^2 \text{Vol}_{\M}. $$
It is a classic fact that a map $f:\M \to \N$ is a critical point of $E$
 if and only if $\delta(df)=0$, where $\delta$ is the adjoint of $\nabla^{f^*{\TN}}$. (such a map $f$ is called harmonic).

I am asking whether or not an analogous characterization of critical points holds for this modified functional:

$$\tilde E(f)=\frac{1}{2}\int_{\M} | \bigwedge^2 df|^2 \text{Vol}_{\M}$$
$\tilde E(f)$ measures the mean action of $f$ on 2$D$-cubes. (How $f$ affects areas of surfaces, locally).

Note that $\bigwedge^2 df\in \Omega^2\big(\M,\Lambda_2(f^*{\TN})\big)$. Let $\nabla^{\Lambda_2(f^*{\TN})}$ be the induced connection on $\Lambda_2(f^*{\TN})$ and let $\delta_{\nabla^{\Lambda_2(f^*{\TN})}} $ be the adjoint of the covariant exterior derivative $d_{\nabla^{(f^*{\TN})}}$.

>I want to know if the set of critical points of $\tilde E$ coincides with set of maps satisfying $\delta_{\nabla^{\Lambda_2(f^*{\TN})}} \big( \bigwedge^2 df \big) =0$.

One direction follows easily from the Euler-Lagrange equation of $\tilde E$: I proved [here][2] that the E-L equation is
$$h_{f^*\TN} \big( \tr_{\TM}\big( df \otimes \delta_{\nabla^{\Lambda_2(f^*{\TN})}}(\bigwedge^2 df)\big) \bigg)=0,$$

where $h_{f^*\TN}:f^*\TN \otimes \Lambda_2(f^*\TN) \to f^*\TN$ is the linear extension of

$$ \tilde w \otimes (w_1 \wedge w_2) \to \langle \tilde w,w_2 \rangle w_1-\langle \tilde w,w_1 \rangle w_2.$$

Thus $\delta_{\nabla^{\Lambda_2(f^*{\TN})}} \big( \bigwedge^2 df \big) =0$ implies $f$ is critical.

>**Question:** Does every critical point of $\tilde E$ satisfy $\delta_{\nabla^{\Lambda_2(f^*{\TN})}} \big( \bigwedge^2 df \big) =0$?

Of course, by counting degrees of freedom, this doesn't look reasonable.



**Edit 1:**

As explained below, in dimension $d=2$ the answer is positive. (Note that in this case the "number of constraints" is the same).  Focusing on the simplest next case, we consider $d=3,\M=\N=\R^3$ with the flat metrics.

**First, let's try to answer a more degenerate version of the question (forgetting $h_{f^*\TN}$):**

>Is there a smooth map $f:\mathbb{R}^3 \to \mathbb{R}^3$, which satisfy

>$$\delta \big( df \wedge df \big) \neq 0, \, \text{and } \,  \tr \big( df \otimes \delta(df \wedge df) \big)=0, $$

If there is such an $f$ then it cannot be a local diffeomorphism; its Jacobian must vanish somewhere.

Here $ \delta \big( df \wedge df \big)\in \Omega^1\big(\R^3;\Lambda_2(\R^3)\big)$ is a one-form on $\R^3$ with values in $\Lambda_2(\R^3)$, and

$$ \delta \big( df \wedge df \big)(X)=\sum_i \nabla_{e_i}(df \wedge df)(e_i,X)=\sum_i (\nabla_{e_i} df)(e_i) \wedge df(X) + df(e_i) \wedge (\nabla_{e_i} df)(X)$$
$$ =\Delta f \wedge df(X)+\sum_i df(e_i) \wedge (\nabla_{e_i} df)(X).$$
  $\nabla_{e_i}$ is the standard component-wise derivative w.r.t the variable $x_i$, or the Levi-Civita connection of $\R^3$, if you prefer.

We can try to look first for harmonic counter-examples, but so far I failed doing even that.

Note that in the second equation we refer to the trace taken w.r.t the Euclidean metric:
$$ \tr \big( df \otimes \delta(df \wedge df) \big)=\sum_i df(e_i) \otimes \bigg( \delta \big( df \wedge df \big)(e_i) \bigg)=0.$$

**Edit 2:**

>The answer is positive for $d=2$, so we need to restrict our attention to $d \ge 3$.

Indeed, in this case $E(f)=\int_{\M}| \bigwedge^2 df|^2  \text{Vol}_{\M}=\int_{\M}(\Det df)^2  \text{Vol}_{\M}$. We shall prove a map $f$ is $E$-critical if and only if its determinant is constant.

Since  $\Det df$ is constant $\iff$ $\nabla (\bigwedge^2 df)=0 \iff \delta_{\nabla^{\Lambda_2(f^*\TN)}} \big( \bigwedge^2 df \big)=0$, we are done.

The Euler-Lagrange equation of $E$ can be written as

$$ \delta (\Det df \cdot \Cof df) =0, \tag{1}$$ where $\Cof df:\TM \to f^*(\TN)$ is the *cofactor map* of $df$ defined by $$ \Cof df=  \star_{f^*TN}^{d-1} (\wedge^{d-1} df) \star_{TM}^1, $$

and $\delta$ is the adjoint of the pullback connection on $f^*\TN$.

(The Euler-Lagrange equation of the Jacobian functional $E(f)=\int_{\M} \Det df \text{Vol}_{\M}=\int_{\M} f^* \text{Vol}_{\N}$ is $\delta (\Cof df)=0$. Details can be found in lemma 2.9 in my paper [here][3]).

Expanding equation $(1)$ we get
$$
  0=\delta(\Det df \cdot \Cof df )= \Det df \delta(\Cof df ) - \tr_{g}(d \Det df \otimes \Cof df).
  $$

We now use the fact the Jacobian functional is a null-Lagrangian, i.e. every smooth map is a critical point, or equivalently $\delta (\Cof df)=0$. (This is essentially Stokes theorem, you can see lemma 2.5 [here][3]). 

So, the E-L equation $(1)$ reduces to $$\tr_{g}(d \Det df \otimes \Cof df)=0. \tag{2}$$

Let $f$ be a map satisfying equation $(2)$. We shall prove $\Det df$ is constant; suppose that $\Det df_p \neq 0$ for some $p \in \M$. This implies $ \Cof (df_p)$ is invertible, so by equation $(2)$ $d\Det df_p=0$. 

Now [we observe][4] that any $C^1$ function $g : \M \to \mathbb R$ on a connected manifold, satisfying $g(p) \ne 0 \implies dg_p = 0$ is constant. 





[1]:https://mathoverflow.net/questions/270836/is-deltadf-wedge-df-0-an-euler-lagrange-equation
[2]:https://mathoverflow.net/a/271264/46290
[3]:https://arxiv.org/abs/1701.08892
[4]:https://math.stackexchange.com/a/2489068/104576