Differences

This shows you the differences between two versions of the page.

--- harmonic_map [2022/08/31 15:13] – [Examples] spencer
+++ harmonic_map [2022/09/01 13:05] (current) – spencer
@@ Line 10: / Line 10: @@
 Let $M = S^1$.
 Then a harmonic map $u : M \to N$ is a closed geodesic; that is, a curve that is both a geodesic and periodic.
-Indeed, working in a local coordinate $x$ on $S^1$ so $u : \mathbb{R} \to N$ in this chart, the harmonic map equation becomes
+Indeed, working in a local coordinate $x$ on $S^1$ and coordinates on $N$ so that $u(x) = (u^1(x),\ldots,u^m(x))$, the harmonic map equation becomes that for each $k$,
-\[ \frac{\partial^2 u}{\partial x^2} - \Gamma^N_{ij}^k \frac{\partial u}{\partial x^i} \frac{\partial u}{\partial x^k} = 0, \]
+\[ \frac{\partial^2 u^k}{\partial x^2} - (\Gamma^N)_{ij}^k \frac{\partial u^i}{\partial x} \frac{\partial u^j}{\partial x} = 0, \]
 which is precisely the geodesic equation.
+=== The identity map ===
+The identity $\mathrm{Id} : (M,g) \to (M,g)$ is always harmonic.
+Indeed in this case, the pushforward $\mathrm{d}\,\mathrm{Id}$ is the identity map $TM \to TM$ and so is given in a frame $\{e_1, \ldots, e_n\}$ with coframe $\{e^1,\ldots, e^n\}$ by $\mathrm{d}\,\mathrm{Id} = e^i \otimes e_i$.
+In particular,
+\[ e(\mathrm{d} Id) = n \]
+is a constant, **TODO**
+===== Second variation formula =====
+See (3.38), Eells & Lemaire.
+**Claim:** If $u_t$ is a family of harmonic maps, then $\left.\frac{\partial u_t}{\partial t}\right\rvert_{t=0}$ is a Jacobi field $J_u$.
+**Proof:**
+We ob