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math214:04-06 [2020/04/06 08:28] pzhou |
math214:04-06 [2020/04/06 10:35] (current) pzhou [Bianchi Identity] |
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$$ \d_t u^\alpha(t) = - [\Gamma_1]^\alpha_\beta(0, | $$ \d_t u^\alpha(t) = - [\Gamma_1]^\alpha_\beta(0, | ||
Approximately, | Approximately, | ||
- | $$ [u](\epsilon) \approx (1 + \epsilon \Gamma_1(0, | + | $$ [u](\epsilon) \approx (1 - \epsilon \Gamma_1(0, |
- | The parallel transport along the first segment is $$P_1 \approx 1 + \epsilon \Gamma_1(0, | + | The parallel transport along the first segment is $$P_1 \approx 1 - \epsilon \Gamma_1(0, |
Similarly, we have | Similarly, we have | ||
- | $$ P_2 \approx 1 + \delta \Gamma_2(\epsilon, | + | $$ P_2 \approx 1 - \delta \Gamma_2(\epsilon, |
Using Taylor expansion for $\Gamma$ at $(0, | Using Taylor expansion for $\Gamma$ at $(0, | ||
- | $$ P_4 P_3 P_2 P_1 \approx (1 - \delta \Gamma_2 ) (1 - \epsilon \Gamma_1 | + | $$ P_4 P_3 P_2 P_1 \approx (1 + \delta \Gamma_2 ) (1 + \epsilon \Gamma_1 |
+ | $$ \approx 1 - \epsilon \delta (\d_1 \Gamma_2 - \d_2 \Gamma_1 + \Gamma_1 \Gamma_2 - \Gamma_2 \Gamma_1)|_{(0, | ||
+ | Hence we are done. See also [Ni] 3.3 for a more rigorous derivation. | ||
+ | |||
+ | ===== Bianchi Identity ===== | ||
+ | $$ \nabla^{\End(E)}(F_\nabla) = 0 $$ | ||
+ | |||
+ | (1) Formal proof. If $T \in \Omega^p(M, \End(E))$, then we have $$\Phi_T: \Omega^k(M, E) \to \Omega^{k+p}(M, | ||
+ | $$ \Phi_{\nabla(T)} = [\nabla, \Phi_T] = \nabla \Phi_T - (-1)^p \Phi_T \nabla $$ | ||
+ | that is, for a section $u \in \Omega^k(M, E)$, we have | ||
+ | $$ [\nabla(T)](u) = \nabla (T \wedge u) - (-1)^p T \wedge (\nabla u). $$ | ||
+ | |||
+ | Now, take $T = F_\nabla \in \Omega^2(M, \End(E))$, we need to show that $\nabla(F_\nabla) = [\nabla, \nabla^2] = 0$. done. | ||
+ | |||
+ | This seems too easy, did I miss a sign? (no..) | ||
+ | |||
+ | (2) Try again, using local presentation | ||
+ | $$ F = (d + A)^2 = dA + A \wedge A = dA + (1/2) [A, A] $$ | ||
+ | where in the last expression $\End(E)$ is viewed as a Lie algebra. | ||
+ | |||
+ | Then | ||
+ | $$ \nabla(F) = [\nabla, F] = [d+A, dA + (1/2) [A, A]] = (1/2) d[A, A] + [A, dA] + (1/2) [A, [A, A]] = (1/2) [A, [A, A]] $$ | ||
+ | The last quantity is zero, by Jacobi identity, to be more explicity, we write | ||
+ | $$A = \sum_i dx^i \ot \Gamma_i, \quad \Gamma_i \in M_n(\R)$$ | ||
+ | Then | ||
+ | $$[A, [A, A]] = \sum_{i, | ||
+ | for example, the term with $dx^1 \wedge dx^2 \wedge dx^3$ has (2 times) | ||
+ | $$ [\Gamma_1, [\Gamma_2, \Gamma_3]] + [\Gamma_2, [\Gamma_3, \Gamma_1]] + [\Gamma_3, [\Gamma_1, \Gamma_2]] = 0. $$ | ||
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