# Rational Polynomial Substitution

**URL:** <https://discourse.julialang.org/t/rational-polynomial-substitution/124820>\
**Category:** Specific Domains\
**Tags:** question\
**Created:** [January 16, 2025, 9:39am UTC](https://discourse.julialang.org/t/rational-polynomial-substitution/124820 "2025-01-16T09:39:49Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![gustave\_Robichon](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/gustave_robichon/32/207522_2.png) [@gustave\_Robichon](https://discourse.julialang.org/u/gustave_Robichon)\
**Post date:** [January 16, 2025, 9:39am UTC](https://discourse.julialang.org/t/rational-polynomial-substitution/124820/1 "2025-01-16T09:39:49Z")

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Hello there 🙂  
I am using some Rational Polynomial object. I want to substitute some variables inside it. How can I do ? Here is the code :

```julia
using DynamicPolynomials
@polyvar eta_var
eta = (0.0 + (1.0+0.0im)*eta_var) / (1.0 + (0.0 + 0.0*1im)*eta_var)
subs(eta , eta_var => inv(eta) ) 

```

the second line is to declare a variable. The third line is to make a rational polynomial. And the fourth one is a try to substiture (which does not work).  
Any idea how to make it work ? Thanks a lot 🙂

The problem is that the substition uses inv(eta), which is not a Polynomial but a Rational Polynomial …

(finally I’ve written it by my way, so that I am not blocked ! But the question is still there 🙂 )

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<div class="post-metadata">

**Author:** ![baggepinnen](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/baggepinnen/32/693_2.png) [@baggepinnen](https://discourse.julialang.org/u/baggepinnen)\
**Post date:** [January 16, 2025, 2:12pm UTC](https://discourse.julialang.org/t/rational-polynomial-substitution/124820/2 "2025-01-16T14:12:54Z")

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If this is all you want to do, `Symbolics.substitute` handles that. Here are some utilities for working with symbolic transfer functions with Symbolics and SymPy

> **[GitHub - JuliaControl/SymbolicControlSystems.jl: C-code generation and an interface between...](https://github.com/JuliaControl/SymbolicControlSystems.jl)**
>
> C-code generation and an interface between ControlSystems.jl and SymPy.jl