# Derivative of a registered function
**URL:** https://discourse.julialang.org/t/derivative-of-a-registered-function/69656
**Category:** Modelling & Simulations
**Tags:** modelingtoolkit, symbolics
**Created:** [October 13, 2021, 3:28am UTC](https://discourse.julialang.org/t/derivative-of-a-registered-function/69656 "2021-10-13T03:28:01Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@PikaPei](https://discourse.julialang.org/u/PikaPei)
#### Post date: [October 13, 2021, 3:28am UTC](https://discourse.julialang.org/t/derivative-of-a-registered-function/69656/1 "2021-10-13T03:28:01Z")
I want to compute some Jacobian matrix and get the numerical value.
However, if I registered the user-defined function, I can’t get the result.
For example, before `@register`:
```julia
using ModelingToolkit
@parameters t
D = Differential(t)
@variables x(t)
f(x) = tanh(x)
```
```julia
julia> A = [f(x)]
1-element Vector{Num}:
tanh(x(t))
julia> jac = Symbolics.jacobian(A, [x])
1×1 Matrix{Num}:
1 - (tanh(x(t))^2)
julia> Symbolics.substitute.(jac, [x => 1])
1×1 Matrix{Num}:
0.41997434161402614
# the value above is what I want to get
```
After `@register`:
```julia
julia> @register f(x)
julia> A = [f(x)]
1-element Vector{Num}:
f(x(t))
julia> jac = Symbolics.jacobian(A, [x])
1×1 Matrix{Num}:
Differential(x(t))(f(x(t)))
julia> Symbolics.substitute.(jac, [x => 1])
1×1 Matrix{Num}:
Differential(x(t))(0.7615941559557649)
```
It seems that `Symbolics.expand_derivatives` is ineffective?
```julia
julia> Symbolics.expand_derivatives.(jac)
1×1 Matrix{Num}:
Differential(x(t))(f(x(t)))
```
I didn’t find much information in the documentation, did I miss something?
Or how can I compute the derivative?
---
### Author:  [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)
#### Post date: [October 13, 2021, 4:04am UTC](https://discourse.julialang.org/t/derivative-of-a-registered-function/69656/2 "2021-10-13T04:04:23Z")
You have to define the derivative. [Derivatives and Differentials · Symbolics.jl](https://symbolics.juliasymbolics.org/dev/manual/derivatives/#Adding-Analytical-Derivatives-1)
---
### Author:  [@PikaPei](https://discourse.julialang.org/u/PikaPei)
#### Post date: [October 13, 2021, 7:03am UTC](https://discourse.julialang.org/t/derivative-of-a-registered-function/69656/3 "2021-10-13T07:03:27Z")
Oh, I didn’t notice that. It works, thank you!