# Bad math, what's it all about?

**URL:** <http://discourse.iapct.org/t/bad-math-whats-it-all-about/1542>\
**Category:** CSG1996\
**Created:** [January 16, 1996, 3:54pm UTC](http://discourse.iapct.org/t/bad-math-whats-it-all-about/1542 "1996-01-16T15:54:03Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![Richard\_Marken](http://discourse.iapct.org/letter_avatar_proxy/v4/letter/r/9fc348/32.png) [@Richard\_Marken](http://discourse.iapct.org/u/Richard_Marken)\
**Post date:** [January 16, 1996, 3:54pm UTC](http://discourse.iapct.org/t/bad-math-whats-it-all-about/1542/1 "1996-01-16T15:54:03Z")

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[From Rick Marken (960116.0800)]

Bruce Abbott (960115.1215 EST) --

Me:

> o := o + (k.2i-o)

Rick:

> o := o + (k.2i-o)

Bruce:

> But doesn't this reduce to just

> o := k.2i ?

Rick:

> Mathematically, yes. But with the replacement operation I think we just end  
> up adding an increment, k.2i-o, to o.

Bill Powers (960116.0500 MST) --

> No matter how you think about it, o + k.2i - o is just k2.i.

> You've left out the slowing factor,

Oops. So my simulation is a proportional rather than an integral control  
system. As I said in the part of the post that you didn't scan because of  
your compiler policy, I think in way over my head (obviously, since I  
didn't notice that o + k.2i - o = k2.i).

I think the relevant points in this discussion are:

1. X(n) = g (Xn-k) does not describe a closed loop system and

2. When loop gain in a positive feedback system is \>= 1.0 the system becomes  
unstable.

Do these statements make it through the first pass?

Best

Rick