# Questions tagged [expression-manipulation]

How to manipulate expressions structurally, not necessarily complying with the rules of algebra.

89 questions with no upvoted or accepted answers
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### Eliminate variable with partially documented Solve feature Solve[eqns,vars,elims]. Robust?

The Eliminating Variables section on Manipulating Equations and Inequalities documentation quotes an otherwise undocumented feature for Solve ...
• 37k
114 views

### Why does the kernel quit if I want to use minus a sum that I converted from a list?

Using Mathematica 11 when I define a list yy = {x*43, 32*y}; and then convert it to a sum yy[[0]] = Plus; yy I get, as ...
• 950
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### Why doesn't Log[Gamma[]] simplify to LogGamma[] where it could?

I have been playing with various equations involving amount of permutations in relatively large sets. Easiest way to look at these is something like Log[10, bignumber!] . Often expressions, even ...
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398 views

### Unpacking list of arguments into parameters

I have a list in the following form: list = {{a,1,3},{b,1,3},{c,1,3}} Now I want to apply this list as parameters for the Sum function so that I end up with the ...
• 131
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### Computations with OptimizedExpressions without completely expanding them

I have to manipulate huge expressions that are rational functions of many (∼30) variables with integer coefficients. Storing them just as a ratio of two polynomials would be impractical. But they can ...
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### Select certain term containing some patterns and confusing pattern Alternatives in FreeQ

I am considering a function termsContain[func_,list_] which select in an expression func the terms containing the ones in the ...
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### How to find where in a large expression Power::infy is generated?

Consider the following expression: ...
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### Bringing rational functions to canonical form

I have a complicated expression, which among other things has stuff like functions of rational functions: expr = f[x/(-x^2 + y)] + f[-(x/(x^2 - y))] (* + ...*) ...
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362 views

### Force expression in certain form

Is there a function in Mathematica to force an expression to be transformed into a given form. For example f[x] is a function of ...
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133 views

### Check an Option without Evaluation

Consider the following: ...
• 753
114 views

### How to combine DifferenceRoot objects for odd and even-indexed terms

I'm trying to analyze a certain infinite sequence $S$, indexed by positive integers starting from $1$. It can be split into two subsequences: $S^{odd}$ by removing all even-indexed elements and ...
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### Don't simplify integer square roots

How can I rewrite an expression like $\frac{1}{2\sqrt{3}}$ as $\frac{1}{\sqrt{12}}$? This works in principle, but gets immediately simplified back to the original expression ...
• 1,441
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### Define quaternions by properties instead of coordinates

I'd like to manipulate an expression of quaternions and only use the following properties ...
• 201
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### How to do an arbitrary expression growth in Mathematica?

I want to write a function that can be used to grow an expression from a seed or another existing expression based on part specification. ...
• 9,651
147 views

### General rule to rewrite function and argument into a new symbol

I am trying to convert Mathematica output into equations I can plug into C. I am presently trying to convert expressions of the form Derivative[1,0,0][f][t,x,y] ...
175 views

### Substitution of expressions using Groebner basis

I know this topic came up numerous times (most of the results are detailed in the comments of this link Is there a way to to substitute for symbols, simplify, and back-substitute?). However, I have ...
• 375
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### Manipulating Equations: Substracting two equations

I was wondering why I fail to substract two equations using thread, while it works properly, as described here for additions. Example: Let a system of equations be given: ...
72 views

### Generating a list of 4-digit integers by simulating four dice being thrown eight times

I simulate throwing four dice and merge their values together to form a 4-digit number. This is done eight times and I want to get a list like this one: ...
• 193
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### Create Association from Expression

I have a set of algebraic expressions of a certain form, and I would like to make them into Associations in a specified way. My expressions are formed of two heads, ...
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270 views

1 vote
63 views

### Finding a selected expression

How can I find all expressions matching the one I am selecting? This features exists in Matlab and is quite helpful: Just trying to make my debugging easier. Thanks!
1 vote
93 views

### Canceling factors from numerator and denominator with complex number

I have the following expression: $\text{expr} = i \frac{\sqrt{a-b}}{\sqrt{b-a}}x$. Clearly simplifying expression gives $\text{expr}=x$ by cancelling the numerator and denominator. However, I am not ...
• 111
1 vote
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### How can I force the output of a computation to be as compact as possible, yet complete, by using functions?

I am computing some derivatives in Wolfram Mathematica 11.3. The functions that I have to differentiate (with respect to the state variables) are the equations of motion of a rocket in the atmosphere. ...
1 vote
223 views

### how to sort an expression with descending coefficients?

I have an expression for example psi = 0.24 FF1[0] FF2[1] FF3[-1] + 0.23 FF1[0] FF2[0] FF3[1] + 0.5 FF1[1] FF2[0] FF3[1] + 0.25 FF1[1] FF2[1] FF3[1] I wonder how ...
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1 vote
51 views

### Do replacement only on certain specific forms

If I want to replace, say, a with a1 + a2 in an expression, I can write ...
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1 vote