# Factor out known multipliers in algebraic expressions?

GROUPS:
 Hi there,I have this function of a[t], and it's a long function of other variables in time and some constant parameter (Ixx, Iyy, Izz), which I really want to explicit in a somatory of any combination of those constant parameters. I've tried to Simplify and then Apart, but Mathematica doesn't group similar coefficients. I wonder if I could get this result applying some kind of mapping (...)?Well, below is the function a[t]: a[t_] := -((Cos[ϕ[t]]* Izz[t]*(Ty[t] - Cos[ϕ[t]]*Iyy[t]*Derivative[1][ϕ][t]* Derivative[1][ψ][t] + Cos[ϕ[t]]*(-Ixx[t] + Izz[t])* (Sin[ψ[t]]*Derivative[1][θ][t] + Derivative[1][ϕ][t])*((-Cos[ψ[t]])* Tan[ϕ[t]]*Derivative[1][θ][t] + Derivative[1][ψ][t]) + Iyy[t]*Derivative[1][θ][ t]*(Cos[ψ[t]]*Sin[ϕ[t]]* Derivative[1][ϕ][t] + Cos[ϕ[t]]*Sin[ψ[t]]* Derivative[1][ψ][t])) - Iyy[t]*Sin[ϕ[t]]*(Tz[t] + Izz[t]*Sin[ϕ[t]]*Derivative[1][ϕ][t]* Derivative[1][ψ][ t] + (Ixx[t] - Iyy[t])*(Sin[ψ[t]]*Derivative[1][θ][t] + Derivative[1][ϕ][t])* (Cos[ϕ[t]]*Cos[ψ[t]]* Derivative[1][θ][t] + Sin[ϕ[t]]*Derivative[1][ψ][t]) + Izz[t]*Derivative[1][θ][ t]*(Cos[ϕ[t]]*Cos[ψ[t]]* Derivative[1][ϕ][t] - Sin[ϕ[t]]*Sin[ψ[t]]* Derivative[1][ψ][t])))/((-Cos[ϕ[t]]^2)* Cos[ψ[t]]*Iyy[t]*Izz[t] - Cos[ψ[t]]*Iyy[t]*Izz[t]*Sin[ϕ[t]]^2)); Summarizing, what I want is a way to say : rewrite a[t] factorizing in a sum by these terms (Ixx, Iyy, Izz) on any combination between them. ( In other way, I could ask the same by rewriting a[t] factorizing in terms that don't vary in time? Would this be possible/easier? )Can anybody suggest a good thing to try?Thanks. Attachments:
4 months ago
6 Replies
 Hans Dolhaine 2 Votes I am not quite sure what you want to do, but first of all in a [ t ] your "constant parameter (Ixx, Iyy, Izz)" are functions of t as well.So I remove this num = Numerator[a[t]] /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz} // Expand den = Denominator[a[t]] /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz} // FullSimplify and Collect terms num1 = Collect[num, {Ixx, Iyy, Izz, Ixx Iyy, Ixx Izz, Iyy Izz}] Check it (a[t] - num1/den /. {Ixx[t] -> Ixx, Iyy[t] -> Iyy, Izz[t] -> Izz}) // Simplify You may well replace Ixx for Ixx [ t ] ans so on in num1 / den again....Hope this helps.
4 months ago
 Thanks for the help. Check the new file that I've uploaded below.
4 months ago
 Thanks for the help! You are right, sorry for the mistake, Ixx[t] is Ixx; e this also aplies to Iyy and Izz.However, I've tried to use it and still isn't what I want as an answer. To put in other example,... Let's say that I have this expression: y = A*Cos [\[Theta]] + (B/A)*Sin[\[Theta]] + (B*A)*x + A*Tan[x] + (B/A)*x^2 And, what I want to know is how to rewrite it like this, using Mathematica methods: y = A*(Cos [\[Theta]]+Tan[x]) + (B/A)*(Sin[\[Theta]]+x^2) + (B*A)*x Because I want "y" as a sum of the constant terms {A,B} in any combination between them, but also multiplying a function that do not include them, like above. For example, this format o y, applying Apart for "B" Apart[ y , {B}] Results in: {(B (A^2 x + x^2 + Sin[\[Theta]]))/A + A (Cos[\[Theta]] + Tan[x])} Which doesn't help, because the first polynomial term contains "A", that I want only as a coefficient.Thanks again for the help. Attachments:
 Hans Dolhaine 1 Vote hmmm - ok. perhaps you should disguise the mixed terms. Seems that this Collect[y /. {A B -> ab, A/B -> adb, B/A -> bda}, {A, B, ab, adb,bda}] /. {ab -> A B, adb -> A/B, bda -> B/A} works in your simple example, but for sure it will get tedious in more complicated cases.And what is wrong with the method I mentioned first? This gives num1 as a polynomial in Ixx, Ixx^2, Ixx * Iyy and so on with coefficients as functions depending only of t. Same for the denominator and then you can write your a [ t ] in the form you want.
 Hans Dolhaine 1 Vote Another method, perhaps easier to be made more general is: Find the "variables" - meaning the expressions not being dependet of A or B vv = Complement[Variables[y], {A, B}] Now find the coefficients of these variables cc = Coefficient[y, #] & /@ vv and then find the factors of these Collect[y, cc]