Questions tagged [indexed-variable]

Questions about indexed variables and related alternative methods to allow assigning indexed names to objects.

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6
votes
1answer
171 views

Best way to write an equation with subscripts and superscripts

This post is not about a specific Mathematica problem but about to develop the best way to enter mathematical equations with subscripts and superscripts. I have a ...
0
votes
1answer
43 views

Derivative not evaluating when function defined

I have the following code ...
1
vote
1answer
57 views

xAct, xTensor: How to avoid clash of indices?

Please refer to the picture below. In the first line, I define the angular momentum vector $\vec{L} = \vec{R} \times \vec{P}$ using the Levi-Civita tensor $\epsilon^{i}_{jk}$. The definition relies on ...
1
vote
2answers
62 views

Collecting terms from expression with indexed functions

Say I have an expansion of terms containing functions y[j,t] and its derivatives, indexed by j with the index beginning at 0 ...
0
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1answer
43 views

Getting named functions from an sequence of parameter values applied to a parametric function [closed]

The general question is how can I loop on names and indices? An example: I have pfun = ParametricNDSolveValue[{y'[t] == a y[t], y[0] == b}, y, {t, 0, 10}, {a,b}] ...
0
votes
2answers
99 views

Summation over an index and a set

I have an index CC and a set of values CM, and c is an element of CM and ...
2
votes
1answer
33 views

Loop with indexed function not defined for some index values [duplicate]

If anyone cares, this is supposed to be a very simple implementation of a Metropolis algorithm for a uniform distribution. I want to plot the variance of the sampled values as a function of an ...
6
votes
1answer
173 views

Einstein summation convention for symbolic vector calculus

I am trying to do some vector calculus in Mathematica in index notation form because it gives a clear result that can be compared to pen and paper calculations. Since there is no built in Einstein ...
0
votes
0answers
39 views

On the variables used by ForAll

I have the following system of inequalities: test = (1+a>=b)&&(a>=0)&&(1+a+b>=0) So if I use the following code: ...
0
votes
2answers
54 views

Sum over integer partition with variable function argument

Define $$\hat{X}(Y) = [X,Y] $$ I have known matrices $S_i$ and $V$. I am trying to use Mathematica to define a function which calculates $$ \sum_{\substack{n_1, \ldots, n_k>1\\ n_1+\ldots n_k = ...
1
vote
1answer
47 views

Create list of values with arbitrary index and the use it in a function

I have the following generating functions: $l_{2i-1}=l_{1}-(i-2)(w+s)$ with $i\geq 2$ and $l_{2i}=l_{2}-(i-1)(w+s)$ with $i\geq 2$, so the first one is for odd index and the second for even index. ...
4
votes
1answer
55 views

Taking a Part of a Part of a Variable

Say I have Part of a variable x. Part[x, 1] Then, I'll get an error message, as the ...
2
votes
2answers
37 views

Naming indexed output streams

I need to write into multiple files in a Do loop. Hence, I tried to open multiple streams: ...
0
votes
1answer
35 views

How to truncate equation with multivariables

I am trying to set certain terms zero. How to set terms with greater than 'a3' and 'an3' zero in the following equation: ...
2
votes
0answers
60 views

ParallelDo with doubly-indexed iterators

What is the reason that ParallelDo works with singly-indexed iterators, but not doubly- or more-indexed iterators? For example, the code ...
1
vote
0answers
49 views

Symbolic summation with variable bounds and variable number of indices

I wish to compute compute terms like Sum[f[t[j[1]],t[j[2]],...],{j[1],m},{j[2],n},...] for arbitrary positive integer n and any ...
0
votes
1answer
50 views

Multivariable functions symmetric in their arguments/representing indexed coefficients

Context: I have equations for indexed coefficients $C_{ij}$ which I have represented using 2-variable functions in Mathematica. e.g., ...
1
vote
1answer
111 views

subscript index notation select certain value [closed]

I have an indexed variable Subscript[p, s, m, t] established using the Notation method. There are various expressions where s, <...
1
vote
1answer
60 views

How do we input powers of indexed variables?

I have tried (a[i])^2 and alternatively (a^2)[i] Apparently both seem to work, but I was wondering what was the right way ...
0
votes
1answer
117 views

How do I get Mathematica to invert a matrix with subscripts involved? (Subscript usage)

I want Mathematica to invert a matrix in which all variables are symbolic including Subscript. Now I am guessing this has something to do with the method I'm ...
3
votes
2answers
289 views

Combining Multiple Lists

Let's say I have 4 lists: list1 = {a,b,c,d}; list2 = {e,f,g,h}; list3 = {i,j,k,l}; list4 = {m,n,o,p}; I can combine them using Join ...
28
votes
5answers
2k views

What are the requirements for a well behaved indexed variable? Subscript, ToExpression, Downvalue?

On writing this answer I needed to call a function, (NonlinearModelFit) with an unknown number of parameters. We have learned that we should not use ...
6
votes
1answer
1k views

When to use indexed variables

Still learning the fundamentals of the language I would like to ask you what advantages there might be in writing something like: a[1] = 2; a[2] = 4; a[3] = "x"; ...