# Use Sign function in JuMP

**URL:** <https://discourse.julialang.org/t/use-sign-function-in-jump/6764>\
**Category:** Optimization (Mathematical)\
**Tags:** question\
**Created:** [October 30, 2017, 2:39am UTC](https://discourse.julialang.org/t/use-sign-function-in-jump/6764 "2017-10-30T02:39:05Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Qifeng](https://avatars.discourse-cdn.com/v4/letter/q/bc8723/32.png) [@Qifeng](https://discourse.julialang.org/u/Qifeng)\
**Post date:** [October 30, 2017, 2:39am UTC](https://discourse.julialang.org/t/use-sign-function-in-jump/6764/1 "2017-10-30T02:39:05Z")

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Dear all,

I’m solving a MINLP problem in JuMP which contains a nonlinear constraint like this:

y = x^2\*sign(x) #both x and y are continuous.

Previously, I used Couenne through AmplNLWriter interface and got an error of “Unrecognized function sign used in nonlinear expression.” Do you know how to fix this problem?

Thanks in advance.

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**Author:** ![leethargo](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/leethargo/32/6004_2.png) [@leethargo](https://discourse.julialang.org/u/leethargo)\
**Post date:** [October 30, 2017, 7:53am UTC](https://discourse.julialang.org/t/use-sign-function-in-jump/6764/2 "2017-10-30T07:53:30Z")

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Did you also try `y == x * abs(x)`?

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**Author:** ![Qifeng](https://avatars.discourse-cdn.com/v4/letter/q/bc8723/32.png) [@Qifeng](https://discourse.julialang.org/u/Qifeng)\
**Post date:** [October 31, 2017, 8:22pm UTC](https://discourse.julialang.org/t/use-sign-function-in-jump/6764/3 "2017-10-31T20:22:15Z")

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Hi @leethargo. The expression y==x\*abs(x) works. Thank you.