# Projecting push forward of a vector

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1. May 11, 2016

### Augbrah

1. The problem statement, all variables and given/known data
Say we have two manifolds N(dim d) and M(dim d-1). Let Φ: M →N be a diffeomorphism where Σ = Φ[M] is hypersurface in N. Let n be unit normal field (say timelike) on Σ and ⊥ projector (in N) defined by:
$$⊥^a_b = \delta^a_b + n^a n_b$$

Where acting on (s, 0) tensor projection operator is: $⊥T^{a_1 a_2 ... a_r}=⊥^{a_1} _{b_1}...⊥^{a_s} _{b_s}T^{b_1 ... b_s}$

How to show that for a vector V at p on M, the pushforward of V satisfies: $$Φ_*(V)=⊥(Φ_*V)$$ (I can do that)

And then generalize to (s, 0) tensor: $$Φ_*(T)=⊥(Φ_*T)$$

2. Relevant equations

3. The attempt at a solution

In component form we would get: $(⊥(Φ_*V))^a = ⊥^a_b (Φ_*V)^b=(\delta^a_b + n^a n_b)(Φ_*V)^b=(Φ_*V)^a + n^an_b(Φ_*V)^b$. So remains to show that the last bit vanishes. The last term vanishes since $a_b$ is normal to $(Φ_*V)^b$, so we're done.

Now to generalize for (s, 0) we have:

$$(⊥(Φ_*T))^{a_1 a_2 ... a_s} = ^{a_1} _{b_1}...⊥^{a_s} _{b_s}(Φ_*T)^{b_1 ... b_s} = (\delta^{a_1}_{b1} + n^{a_1} n_{b_1}) ... (\delta^{a_s}_{b_s} + n^{a_s} n_{b_s})(Φ_*T)^{b_1 ... b_s}$$

My problem is that I'm not quite sure what does it mean to act $n_{b_s} (Φ_*T)^{b_1 ... b_s}$, notion of orthogonality is easily understood for two vectors, but for vector and a tensor I'm not sure. Any ideas much appreciated!

2. May 11, 2016

### strangerep

Would it help to think of the tensor in terms of (a sum of) tensor products of basis vectors?