# Use Sign Function in JuMP NLconstraint
**URL:** https://discourse.julialang.org/t/use-sign-function-in-jump-nlconstraint/33973
**Category:** Optimization (Mathematical)
**Tags:** question
**Created:** [January 30, 2020, 11:46am UTC](https://discourse.julialang.org/t/use-sign-function-in-jump-nlconstraint/33973 "2020-01-30T11:46:20Z")
**Posts on this page:** 3
**Page:** 1
### Author:  [@cupcc](https://discourse.julialang.org/u/cupcc)
#### Post date: [January 30, 2020, 11:46am UTC](https://discourse.julialang.org/t/use-sign-function-in-jump-nlconstraint/33973/1 "2020-01-30T11:46:20Z")
I’m currently trying to solve a problem in JuMP that has a nonlinear constraint that has the form
y= (other expressions) + constant \* sign(x). This throws an error saying sign is not a recognized function. How do I fix this? (x can take the value of 0 as well, both x and y are continuous)
Thank you!
---
### Author:  [@frapac](https://discourse.julialang.org/u/frapac)
#### Post date: [January 30, 2020, 2:04pm UTC](https://discourse.julialang.org/t/use-sign-function-in-jump-nlconstraint/33973/2 "2020-01-30T14:04:41Z")
Dear @cupcc,
the function `sign ` is currently not implemented in current JuMP’s AD.
I recommend reformulating your problem by using:
`sign(x) = abs(x) / x`
as the function `abs` is supported by JuMP. Your constraint would rewrite:
```julia
y * x = (other expressions) * x + constant * abs(x)
```
(better to avoid divisions in constraints when using a NLP solver)
---
### Author:  [@cupcc](https://discourse.julialang.org/u/cupcc)
#### Post date: [January 30, 2020, 2:15pm UTC](https://discourse.julialang.org/t/use-sign-function-in-jump-nlconstraint/33973/3 "2020-01-30T14:15:06Z")
Thank you so much for the help!