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I have a long equation, namely,

equ = 4576.66 a[1] Cos[t] + 3.3877 a[1]^3 Cos[t]^3 + 4576.65 a[2] Cos[2 t] + 10.1631 a[1]^2 a[2] Cos[t]^2 Cos[2 t] + 10.1631 a[1] a[2]^2 Cos[t] Cos[2 t]^2 + 3.3877 a[2]^3 Cos[2 t]^3 + 4576.63 a[3] Cos[3 t] + 10.1631 a[1]^2 a[3] Cos[t]^2 Cos[3 t] + 20.3262 a[1] a[2] a[3] Cos[t] Cos[2 t] Cos[3 t] + 10.1631 a[2]^2 a[3] Cos[2 t]^2 Cos[3 t] + 10.1631 a[1] a[3]^2 Cos[t] Cos[3 t]^2 + 10.1631 a[2] a[3]^2 Cos[2 t] Cos[3 t]^2 + 3.3877 a[3]^3 Cos[3 t]^3 - 0.415 a[1] Sin[t] - 0.83 a[2] Sin[2 t] - 1.245 a[3] Sin[3 t] == 2/625 Cos[theta] Cos[t w] - 2/625 Sin[theta] Sin[t w]

Since Sin[t]*Cos[t]^3 and so on is a small term, we can take it as 0. Consequently, we would like to eliminate sin[t] and cos[t] to the power of n. My instinct is to use Cases to achieve it, but I don't how to make it.

For example, I want keep 4576.66 a[1] Cos[t] and 4576.65 a[2] Cos[2 t], but set 4576.65 a[2] Cos[2 t] and 10.1631 a[1]^2 a[2] Cos[t]^2 Cos[2 t] to 0.

More Information

  1. Aside from Cos[t]^2 and Cos[2t]^3, Sin[t]*Cos[t] and Sin[t]^2*Sin[2t] and so on should be eliminate. In other words, only Sin[t] Sin[2t] Sin[3t]... Sin[n*t] and Cos[t] Cos[2t] Cos[3t]... Cos[n*t] should be left.

  2. Terms like Sin[theta] Sin[t w] should survive, because Sin[theta] is a constant.

Summary of the solution

Use the following wolfram language grammar to extract the expected term: _ h any expression with head h. Please refer to Patterns and Transformation Rules

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  • $\begingroup$ equ/.{ Cos[_]^2->0,Cos[_]^3->0} will set squares and cubes of Cosine equal to zero. equ/.{ Cos[_]^_->0} will set all powers of Cosine equal to zero, where Cos[x] is not recognized as Cos[x]^1 and thus will not be changed. Can you use this to accomplish all you want? $\endgroup$ – Bill Aug 20 '20 at 7:13
  • $\begingroup$ @Bill Your solution suggestive, but it can not solve my problem completely. Please refer to the More Information in the main post. $\endgroup$ – PureLine Aug 20 '20 at 7:37
  • $\begingroup$ Should terms like Sin[theta] Sin[t w] survive? $\endgroup$ – Natas Aug 20 '20 at 8:27
  • $\begingroup$ @Natas Yes, because Sin[theta] is a constant. $\endgroup$ – PureLine Aug 20 '20 at 8:37
  • $\begingroup$ Since you want set powers of both Sin and Cos to zero I am guessing the parameters a[_] are small? Perhaps then equ /. {Power[a[_], exp_ /; exp >= 2] :> 0, HoldPattern[Times[a[_], a[_]]] :> 0} would work for you? $\endgroup$ – Hausdorff Aug 20 '20 at 8:57
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equ /. HoldPattern[ Power[_Cos | _Sin, _] | 
   Times[(Cos | Sin)[Except[theta]], (Cos | Sin)[Except[theta]], ___]] -> 0

enter image description here

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  • $\begingroup$ Thanks. I got your point, and I add the new knowledge at the end of my main post. $\endgroup$ – PureLine Aug 20 '20 at 14:44

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