Hi,

I’m quite new to Julia, and currently trying out the gradient provided by ForwardDiff.jl. I tried using it on a function that calls `taylorinteg`

from the TaylorIntegration.jl package ; I was expecting it to give a weird result, or no result at all, but I get an error I’m unable to solve. Is it impossible or useless to try to get a result this way ?

Here is a MWE :

```
using TaylorIntegration, ForwardDiff
function deriv_x!(dx, x, params, t)
dx = x
end
function f(x_start)
t, x = taylorinteg(deriv_x!, x_start, 0.0, 10.,
15, 1.0e-15, maxsteps=10_000)
return x[end,:]
end
∇f(x) = ForwardDiff.jacobian(f, x)
print(∇f([1, 2, 3]))
```

And here is the error message :

```
ERROR: MethodError: no method matching setindex!(::Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}, ::ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Float64, 3}, ::Int64)
Closest candidates are:
setindex!(::Taylor1{T}, ::T, ::Int64) where T<:Number at /home/alseidon/.julia/packages/TaylorSeries/vTIIX/src/auxiliary.jl:86
setindex!(::Taylor1{T}, ::Vector{T}, ::StepRange{Int64, Int64}) where T<:Number at /home/alseidon/.julia/packages/TaylorSeries/vTIIX/src/auxiliary.jl:99
setindex!(::Taylor1{T}, ::T, ::StepRange{Int64, Int64}) where T<:Number at /home/alseidon/.julia/packages/TaylorSeries/vTIIX/src/auxiliary.jl:97
...
Stacktrace:
[1] jetcoeffs!(eqsdiff!::typeof(deriv_x!), t::Taylor1{Float64}, x::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, dx::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, xaux::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, params::Nothing)
@ TaylorIntegration ~/.julia/packages/TaylorIntegration/4IefH/src/explicitode.jl:82
[2] __jetcoeffs!(#unused#::Val{false}, f::Function, t::Taylor1{Float64}, x::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, dx::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, xaux::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, params::Nothing)
@ TaylorIntegration ~/.julia/packages/TaylorIntegration/4IefH/src/explicitode.jl:101
[3] taylorstep!(f!::Function, t::Taylor1{Float64}, x::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, dx::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, xaux::Vector{Taylor1{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, abstol::Float64, params::Nothing, parse_eqs::Bool)
@ TaylorIntegration ~/.julia/packages/TaylorIntegration/4IefH/src/explicitode.jl:237
[4] taylorinteg(f!::Function, q0::Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}, t0::Float64, tmax::Float64, order::Int64, abstol::Float64, params::Nothing; maxsteps::Int64, parse_eqs::Bool)
@ TaylorIntegration ~/.julia/packages/TaylorIntegration/4IefH/src/explicitode.jl:478
[5] f
@ ./REPL[3]:2 [inlined]
[6] vector_mode_dual_eval!
@ ~/.julia/packages/ForwardDiff/PBzup/src/apiutils.jl:37 [inlined]
[7] vector_mode_jacobian(f::typeof(f), x::Vector{Int64}, cfg::ForwardDiff.JacobianConfig{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3, Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}})
@ ForwardDiff ~/.julia/packages/ForwardDiff/PBzup/src/jacobian.jl:148
[8] jacobian(f::Function, x::Vector{Int64}, cfg::ForwardDiff.JacobianConfig{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3, Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}, ::Val{true})
@ ForwardDiff ~/.julia/packages/ForwardDiff/PBzup/src/jacobian.jl:21
[9] jacobian(f::Function, x::Vector{Int64}, cfg::ForwardDiff.JacobianConfig{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3, Vector{ForwardDiff.Dual{ForwardDiff.Tag{typeof(f), Int64}, Int64, 3}}}) (repeats 2 times)
@ ForwardDiff ~/.julia/packages/ForwardDiff/PBzup/src/jacobian.jl:19
[10] ∇f(x::Vector{Int64})
@ Main ./REPL[4]:1
[11] top-level scope
@ REPL[5]:1
```

when I paste this in the REPL.

Thanks !

EDIT : changed ForwardDiff.gradient to ForwardDiff.jacobian, as we are in 2 dimensions.