# "x" Argument in Optimization.OptimizationProblem

**URL:** <https://discourse.julialang.org/t/x-argument-in-optimization-optimizationproblem/92971>\
**Category:** Optimization (Mathematical)\
**Tags:** question\
**Created:** [January 14, 2023, 10:09pm UTC](https://discourse.julialang.org/t/x-argument-in-optimization-optimizationproblem/92971 "2023-01-14T22:09:50Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![jonlym](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/jonlym/32/20007_2.png) [@jonlym](https://discourse.julialang.org/u/jonlym)\
**Post date:** [January 14, 2023, 10:09pm UTC](https://discourse.julialang.org/t/x-argument-in-optimization-optimizationproblem/92971/1 "2023-01-14T22:09:51Z")

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The docs for [Optimization.OptimizationProblem](https://docs.sciml.ai/Optimization/stable/API/optimization_problem/) refer to `u` and `u₀` but neither are included in the argument list. Should the second positional argument (i.e., `x`) be relabeled to `u`? If they’re not representing the same thing, what does `x` represent?

I know this question is minor but I wanted to make sure I understood the function correctly.

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**Author:** ![ChrisRackauckas](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/chrisrackauckas/32/77_2.png) [@ChrisRackauckas](https://discourse.julialang.org/u/ChrisRackauckas)\
**Post date:** [January 16, 2023, 3:49am UTC](https://discourse.julialang.org/t/x-argument-in-optimization-optimizationproblem/92971/2 "2023-01-16T03:49:48Z")

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That’s a typo, should be `u0`.