Hello all,

I’m trying to familiarize myself with JuMP, especially in the context of using SpecialFunctions.

I’m struggling with `gamma()`

and `lgamma()`

specifically, so I’ve tried to implement the simple example from JuMP.jl issue #407 albeit with updated syntax:

```
using JuMP
using Ipopt
using SpecialFunctions
m = Model(with_optimizer(Ipopt.Optimizer))
@variable(m, 3.0 <= x <= 4.0)
@NLobjective(m, Max, lgamma(x))
optimize!(m) # expect lgamma(4.0)
```

Running this results in:

```
UndefVarError: digamma not defined
```

I notice that I get the same error if I do not use the `SpecialFunctions`

module, but in that case I haven’t been able to find from where `lgamma()`

is made available.

Package versions here are:

`JuMP v0.20.0`

`Ipopt v0.6.0`

`SpecialFunctions v0.7.2`

`Calculus v0.5.0`

I feel that I’m missing something basic. Am I overlooking a module dependency or not understanding the functions which may be used in a nonlinear objective function? I’ve read through the nonlinear modeling documentation but I am inexperienced in this domain, so any links to relevant documentation are much appreciated.