Order of a pole

Question

Is there a simple way to determine the order of the poles of a rational function?

I have a difficult function where I need Mathematica to find the poles. It would be interesting to also know what the order is. Does there exist a basic command?

It'd be nice if you posted exactly what your "difficult function" looks like. — J. M.'s missing motivation, Jul 27 '15 at 12:44
@J. M. I really don't think it would be illuminating if I explicitly gave you this function. Also I just wanted to know if this was possible in general. — cherzieandkressy, Jul 27 '15 at 13:10
Well, as noted, rational functions are easy. Transcendentals will require some work, since nothing's built-in. — J. M.'s missing motivation, Jul 27 '15 at 13:11
Related: http://mathematica.stackexchange.com/questions/26890/finding-poles-of-a-function — Michael E2, Aug 09 '15 at 12:57

score 11 · Accepted Answer · answered Jul 27 '15 at 13:19

11

One quick way for rational functions is to leverage built-in control system functions:

TransferFunctionPoles[TransferFunctionModel[1/(1 + s^2), s]][[1, 1]]
   {-I, I}

answered Jul 27 '15 at 13:19

J. M.'s missing motivation

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This only gives me the poles, not the order... – cherzieandkressy Jul 27 '15 at 13:24
1

You can use Tally[] for that. ;) – J. M.'s missing motivation Jul 27 '15 at 13:29
1

Ah nice, I understand. TransferFunctionPoles will give the solution multiple times in contrast to Solve. Thanks! – cherzieandkressy Jul 27 '15 at 13:34

Sumit · Answer 2 · 2015-07-27T17:03:21.077

8

Yo can go by solving the inverse of the function and then Count the solution.

f[x_] := x/(a - x)^3/(b - x);
div = x /. Solve[1/f[x] == 0, x];
poles = Union[div]
Count[div, #] & /@ poles

{a,b}

{3,1}

Depending on how complicated your function is, you may have to go for numerical treatment, like NSolve or NRoots.

Tally

As Guess who it is suggests, You can use Tally[div] instead of Count.

For some reason I always forget the right command at right time :] .

edited Jul 27 '15 at 17:03

answered Jul 27 '15 at 12:53

Sumit

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Thanks! This works. As alternatives you mean Reduce? – cherzieandkressy Jul 27 '15 at 13:16
1

You could, of course, use Solve[]/Reduce[] on Denominator[f[x]]. – J. M.'s missing motivation Jul 27 '15 at 13:20
1

@cherzieandkressy, if your function is too heavy for Solve then you may have to go numeric, like NSolve or NRoots. – Sumit Jul 27 '15 at 16:53

Order of a pole

2 Answers2