# 1 \* X' \* X is not symmetric

**URL:** https://discourse.julialang.org/t/1-x-x-is-not-symmetric/11444
**Category:** General Usage
**Tags:** linearalgebra
**Created:** [June 5, 2018, 1:40pm UTC](https://discourse.julialang.org/t/1-x-x-is-not-symmetric/11444 "2018-06-05T13:40:42Z")
**Posts on this page:** 1
**Showing post:** 9

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### Author: ![Tamas\_Papp](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/tamas_papp/32/25949_2.png) [@Tamas\_Papp](https://discourse.julialang.org/u/Tamas_Papp)
#### Post date: [June 5, 2018, 1:59pm UTC](https://discourse.julialang.org/t/1-x-x-is-not-symmetric/11444/9 "2018-06-05T13:59:30Z")

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> [@e3c6](#):
>
> I want to understand where it comes from to understand the kind of floating point errors I might be committing here.

A good starting point would be

> [@PSA: floating-point arithmetic](https://discourse.julialang.org/t/psa-floating-point-arithmetic/8678):
>
> Sometimes people are surprised by the results of floating-point calculations such as julia\> 5/6 0.8333333333333334 # shouldn't the last digit be 3? julia\> 2.6 - 0.7 - 1.9 2.220446049250313e-16 # shouldn't the answer be 0? These are not bugs in Julia. They’re consequences of the IEEE-standard 64-bit binary representation of floating-point numbers that is burned into computer hardware, which Julia and many other languages use by default. Brief explanation You can t…

TL;DR: order matters, if you know (from theory) that two things are equal and want to use it like that, don’t calculate it twice, or discard one of them. Hence `Symmetric`.

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