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Re: remove higher orders terms from mathemtica's result

  • To: mathgroup at smc.vnet.net
  • Subject: [mg90926] Re: [mg90916] remove higher orders terms from mathemtica's result
  • From: Andrzej Kozlowski <akoz at mimuw.edu.pl>
  • Date: Wed, 30 Jul 2008 03:51:27 -0400 (EDT)
  • References: <200807291009.GAA24445@smc.vnet.net>

On 29 Jul 2008, at 12:09, kem wrote:

> Hi,
> Given this:
>
> K = (1+vy) (zx+wx)  + vx (zy+wy);
> Expand[K^2]
>
> Is it possible to remove automatically the 3rd and 4th orders from the
> result of  'Expand[K^2]' ?
>
> By 3rd order I mean terms like: 2 wy zy vx^2  etc. (all terms with sum
> of the degrees of derivatives of u,v,w is = 3)
> By 4th order:  wy^2 vx^2  etc.   (sum of degrees of u,v,w = 4)
>
>
> Thanks
>
>
>
>


Actually, there are many ways, but no single function that would do  
this. Here are a couple of short programs, which get rid of terms od  
degree >= 2

K = (1 + v y) (z x + w x) + v x (z y + w y);

Block[{vars = Variables[K], t},
  Normal[(K /. Thread[vars -> t vars]) + O[t]^3] /. t -> 1]
  w x + z x

Block[{vars = Variables[K], m},
  m = Take[CoefficientArrays[K, vars] // Normal, 3];
  m[[1]] + m[[2]].vars + m[[3]].vars.vars]

  w x + z x

Andrzej Kozlowski


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