# Jacobi Fields

A smooth tangent vector field $J: [0,1] → T\mathcal M$ along a geodesic $g(⋅;x,y)$ is called *Jacobi field* if it fulfills the ODE

where $R$ is the Riemannian curvature tensor. Such Jacobi fields can be used to derive closed forms for the exponential map, the logarithmic map and the geodesic, all of them with respect to both arguments: Let $F:\mathcal N → \mathcal M$ be given (for the $\exp_x⋅$ we have $\mathcal N = T_x\mathcal M$, otherwise $\mathcal N=\mathcal M$) and denote by $ξ_1,…,ξ_d$ an orthonormal frame along $g(⋅;x,y)$ that diagonalizes the curvature tensor with corresponding eigenvalues $κ_1,…,κ_d$. Note that on symmetric manifolds such a frame always exists.

Then $DF(x)[η] = \sum_{k=1}^d \langle η,ξ_k(0)\rangle_xβ(κ_k)ξ_k(T)$ holds, where $T$ also depends on the function $F$ as the weights $β$. The values stem from solving the corresponding system of (decoupled) ODEs.

Note that in different references some factors might be a little different, for example when using unit speed geodesics.

The following weights functions are available

`Manopt.adjoint_Jacobi_field`

— Function```
Y = adjoint_Jacobi_field(M, p, q, t, X, β)
adjoint_Jacobi_field!(M, Y, p, q, t, X, β)
```

Compute the AdjointJacobiField $J$ along the geodesic $γ_{p,q}$ on the manifold $\mathcal M$ with initial conditions (depending on the application) $X ∈ T_{γ_{p,q}(t)}\mathcal M$ and weights $β$. The result is a vector $Y ∈ T_p\mathcal M$. The main difference to `jacobi_field`

is the, that the input `X`

and the output `Y`

switched tangent spaces. The computation can be done in place of `Y`

.

For detais see `jacobi_field`

`Manopt.jacobi_field`

— Function```
Y = jacobi_field(M, p, q, t, X, β)
jacobi_field!(M, Y, p, q, t, X, β)
```

compute the Jacobi jield $J$ along the geodesic $γ_{p,q}$ on the manifold $\mathcal M$ with initial conditions (depending on the application) $X ∈ T_p\mathcal M$ and weights $β$. The result is a tangent vector `Y`

from $T_{γ_{p,q}(t)}\mathcal M$. The computation can be done in place of `Y`

.

**See also**

`Manopt.βdifferential_exp_argument`

— Method`βdifferential_exp_argument(κ,t,d)`

weights for the `jacobi_field`

corresponding to the differential of the geodesic with respect to its start point $D_X \exp_p X[Y]$. They are

**See also**

`Manopt.βdifferential_exp_basepoint`

— Method`βdifferential_exp_basepoint(κ,t,d)`

weights for the `jacobi_field`

corresponding to the differential of the geodesic with respect to its start point $D_p \exp_p X [Y]$. They are

**See also**

`Manopt.βdifferential_geodesic_startpoint`

— Method`βdifferential_geodesic_startpoint(κ,t,d)`

weights for the `jacobi_field`

corresponding to the differential of the geodesic with respect to its start point $D_x g(t;p,q)[X]$. They are

Due to a symmetry agrument, these are also used to compute $D_q g(t; p,q)[η]$

**See also**

`differential_geodesic_endpoint`

, `differential_geodesic_startpoint`

, `jacobi_field`

`Manopt.βdifferential_log_argument`

— Method`βdifferential_log_argument(κ,t,d)`

weights for the JacobiField corresponding to the differential of the logarithmic map with respect to its argument $D_q \log_p q[X]$. They are

**See also**

`Manopt.βdifferential_log_basepoint`

— Method`βdifferential_log_basepoint(κ,t,d)`

weights for the `jacobi_field`

corresponding to the differential of the geodesic with respect to its start point $D_p \log_p q[X]$. They are

**See also**

`differential_log_argument`

, `differential_log_argument`

, `jacobi_field`