Mathematica at WWU

notes by Branko Ćurgus

Mathematica at WWU

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These notes are written for which is available in BH 215. This is the latest version that we have. The current version of Mathematica is v.10.

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Getting started

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• You need to find a campus computer with Mathematica 8 installed on it (for example computers in BH 215). You will find Mathematica 8 as follows (this sequence might differ on different campus computers)
           Start -> Programs -> Math Applications -> Mathematica 8
• When Mathematica starts up, it gives you a blank Mathematica file. A Mathematica file is called a notebook. As you start writing in a blank notebook Mathematica forms a cell; a thin blue frame will appear on the right of the notebook window. This is a cell bracket.
• Start writing something simple, 2+2 for example. After writing 2+2, hold down the Shift key and press Enter (this is abbreviated as Shift+Enter). This tells Mathematica to process your input. The result will appear in an output cell. Notice that the thin blue brackets of the input and output cells have differently designed tops.
• To add a new cell place the pointer in the notebook window below the last cell. Make sure that the pointer becomes a horizontal I-beam. Then click and a cell insertion bar will appear. Only then start typing. By default, new cells are input cells. To change the style of a cell: click the cell bracket; this highlights the bracket; then choose style from the menu item Format -> Style
  
  It is essential that there is no text in an input cell. Text and commands cannot be mixed.

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Selected videos from wolfram.com

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A good place to start with learning Mathematica are the following videos from wolfram.com. Please read my comments as well. One general comment: In the movies below please ignore everything that relates to free-form input. Your homework notebooks should not contain free-form linguistic input. Only Mathematica language is allowed for calculations on Assignments.

• First 10 minutes with Mathematica 8. Pay attention to these features in this movie:
  • 00:45-02:50   Ignore free-form input.
  • 03:23-04:30   Three simple rules of Mathematica language:
    1. Functions in Mathematica are capitalized.
    2. Use [ ] to surround function arguments.
    3. Use { } to denote lists and ranges.
    Three important examples are given here: Plot, Factor and Table. I do not use Factor much; I use Simplify and FullSimplify much more often.
  • 05:50-06:43   How to use Manipulate.
  • 07:15-07:54   How to create Text Cells, Title. Also learn how to make Sections, Subsections, Subsubsections.
  • 08:31-10:05   How to use Mathematica documentation.
  • 10:06-end   Ignore free-form input.
• Movies below are Wolfram movies posted on YouTube. The reason for linking to YouTube is that I liked these movies and I could not find them at the Wolfram site. However, you can find many more useful movies at the Wolfram site Getting Started with Mathematica. Have in mind that the current version of Mathematica is v.10, while we have v.8.
• Hands-on start to Mathematica: Notebooks. Pay attention to these features in this movie:
  • 0:52-2:35   How to create a new notebook and structure it right. This is exactly the form that you should use for your homework notebooks. The title will be Assignment 1, the subtitle is your first and last name, Sections: Problem 1, Problem 2 and so on.
  • 2:35-3:12   How to open and close groups of cell.
• Hands-on start to Mathematica: Methods to get started. Pay attention to these features in this movie:
  • 0:00-1:01   How to organize your notebook. Exactly as what you need for Assignments.
  • 01:00-03:36   Ignore free-form input.
  • 3:39-4:30   Four basic rules of Mathematica language, three as listed above and Shift-Enter rule.
  • 4:30-5:35   Examples of commands in Mathematica language.
  • 5:40-7:35   An ode to Mathematica documentation.
  • 7:40-9:40   Using pallets and keyboard input to write in Mathematica language.
• Hands-on start to Mathematica: Basic calculations. Pay attention to these features in this movie:
  • 0:45-2:35   Mathematica always performs exact calculations. To get approximations use N[]. But please, in your notebooks do not use %, like in N[%12,10], to cite results of previous calculations. This is not a prudent programming practice, since when you reopen your notebook later these numbers will change, so N[%12,10] can mean something completely different.
  • 2:45-3:25   Notice that a matrix in Mathematica is just a list of lists.
  • 3:35-5:00   A variable assignment, much better way to deal with important special values.
  • 5:20-6:50   How to create your own functions. Pay attention to the underscore and colon in the definition of

    f[x_]:=x^2+x+1

    My recommendation is to use more complicated names for functions to avoid duplication and to always clear the string that you are using as the name. So, a better way here would be to write

    Clear[myf,x]; myf[x_]:=x^2+x+1

  • 6:50-7:50   Solving equations: Pay attention to different meaning of these three symbols = (variable assignment), := (set delayed, used when defining functions), == (used in equations to be solved).
• Hands-on start to Mathematica: Basic graphics. Pay attention to these features in this movie:
  • 00:25-04:15   Ignore free-form input.
  • 4:15-4:55   Graphing two or more functions is explained here. Pay attention: two functions are placed in a list.
  • 4:55-6:00   Naming and showing graphics together and using the function Show[]
  • 6:00-8:05   Using Options[Plot] is a very important part of taking full control of your plots.
• Hands-on start to Mathematica: Making interactive models. Pay attention to these features in this movie:
  • 0:40-3:42   Introduction to Manipulate[]. Pay attention to the color of what you type and the matching of brackets.
• Hands-on start to Mathematica: Complete example. Pay attention to these features in this movie:
  • 00:00-00:45   Ignore free-form input.
  • 0:45-1:25   Making a list using Table[] is a very important tool. The list is named data1 and plotted using ListPlot[].
  • 1:30-2:40   The function Fit[] is used to find the line which is the best fit for the data. Then, the data and the line are shown on one plot.

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Brackets in Mathematica

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Before presenting a few examples of how to use Mathematica it is important to notice that there are five types of brackets in Mathematica:

• ( ) -- parentheses or round brackets -- for grouping terms --   ((E^(I Pi)) + 1)
• [ ] -- square brackets or box brackets -- for enclosing arguments of functions --   Cos[Pi/5]
• { } -- braces or curly brackets -- use them to put things together in a list --   {1, 0, Pi, E, I}
• [[ ]] -- double square brackets -- use them with lists --   {1, 0, Pi, E, I}[[4]]
            (The double square brackets will appear nicer if you type   Esc[[Esc   and   Esc]]Esc.)
• (* *) -- starred parentheses -- for comments in Mathematica code --   (* this is a comment *)

Each example below uses some (or all) of these brackets. Please pay attention and make each example a learning experience.

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What to do if things go wrong?

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Eventually things will go wrong. Mathematica might stop responding. When this happens, there are 6 ordered actions that you can do to remedy things. Start with the first action, then proceed to the next one until Mathematica responds.

1. Abort evaluation in the Evaluation menu; the keyboard shortcut is Alt+.
   (This means: hold down the Alt key, then press the . key shortly, and release the Alt key.)
2. In Evaluation menu chose Quit kernel, then click Local.
3. Activate the Windows Task Manager by the keyboard shortcut Ctrl+Alt+Del, then select Processes tab. Highlight MathKernel.exe in the Processes list and click End Process.
4. Exit the Mathematica notebook in the File menu; the keyboard shortcut is Ctrl+F4.
5. Exit Mathematica in the File menu; the keyboard shortcut is Alt+F4
6. Activate the Windows Task Manager by the keyboard shortcut Ctrl+Alt+Del, then highlight Mathematica in the Applications list and click End Task.
7. Power off your computer

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Random, but important remarks

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• Pay attention to the difference between ; and ,. For example, if your cell contains several items, then the items should be separated by ; (not ,). To separate elements of a list one uses ,, as in the above example {1, 0, Pi, E, I}.

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Mathematica has a lot of shortcuts

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Here I list shortcuts that I use often.

• With the cursor in a cell,  Shift+Enter  evaluates that cell. The  Enter  key on the numeric pad is equivalent to  Shift+Enter 
• Just  Enter  starts a new line in a cell.
• Use  Ctrl+6  to write exponents
• Use  Ctrl+2  to write square root
• Use  Ctrl+/  to write fractions
• Use  Esc+[[+Esc  to write left double square bracket
• Use  Esc+]]+Esc  to write right double square bracket
• Use  Alt+.  to stop evaluation
• Use  Alt+;  to spell check spelling
• Use  Ctrl+F6  to cycle through the open notebooks
• Use  Alt+4  to open a section title cell
• Use  Alt+5  to open a subsection title cell
• Use  Alt+6  to open a subsubsection title cell
• Use  Alt+7  to open a text cell

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Arithmetic in Mathematica

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• You can use Mathematica as a calculator. For example to get the circumference of a circle circumscribed about a square with side 1, you type

  Pi*2^(1/2)

  and Shift+Enter.
• To get a numeric approximation type

  N[Pi*2^(1/2)]

  and, as always, Shift+Enter.
• For more see Mathematica's Arithmetic page.

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Mathematical functions and their explorations

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• Mathematica has hundreds of built in functions. Function names are always capitalized, for example the standard ones

  Exp[x], Log[x], Sin[x], Cos[x], Abs[x], ...

  or some which are for some mysterious reason absent from our curriculum:

  Sign[x], Floor[x], Ceiling[x], UnitStep[x], ...

  Please see the menu item
           Help -> Built-in Functions -> Mathematical Functions
• I view all Mathematica commands as functions with several variables. Some examples are below.
• To plot one period of the sine function

  Plot[Sin[x],{x,0,2*Pi}]

  Here Sin[x] is one variable, and the domain {x,0,2*Pi} is another variable. But this function can have many more variables. Options[Plot] will give you all.
• Plotting two functions with several options:

  Plot[
      {Sin[x],Cos[x]}, {x,0,2*Pi},
        PlotStyle->{{Thickness[0.007],Blue},{Thickness[0.007],Green}},
        PlotRange->{-2,2}
      ]

• Plotting a function of two variables:

  Plot3D[ Sin[x]*Cos[y], {x,0,2*Pi}, {y,0,2*Pi} ]

  plots a function of two variables.
• Find a derivative

  D[Sin[3 x]^2 + Cos[4 x], x]

  finds the derivative with respect of x.

• Simplify[D[Sin[3 x]^2 + Cos[4 x], x]]

  finds an algebraically simpler form of the previous answer. Sometimes FullSimplify[] gives more desirable results.

• Integrate[-4 Sin[4 x] + 3 Sin[6 x], x]

  finds an antiderivative.

• Table[Sin[x + s],{s, 0, 2 Pi, Pi/20}]

  produces a list of forty-one shifts of the sine function.
• To plot all the functions from the previous item we need to use Evaluate[]:

  Plot[ Evaluate[Table[Sin[x + s],{s, 0, 2 Pi, Pi/20}]], {x, 0 , 2 Pi} ]

• Notice that exchanging the positions of Plot[] and Table[] and replacing Table[] with Manipulate[] gives a different but useful result:

  Manipulate[
     Plot[Evaluate[Sin[x + s]], {x, 0 , 2 Pi},
        PlotRange->{{0 , 2 Pi},{-1.2,1.2}}] ,
       {s, 0, 2 Pi, Pi/20, ControlPlacement -> Top} ]

• Often it is useful to define our own functions. It is a good practice to clear the name that you use for the function and the name of the variable before the definition:

  Clear[ff,x];
      ff[x_]:= 1- 2 Exp[(x^2)(Cos[x]-1)]

  But much more complicated functions are common. For example functions that include graphing commands.
• The zeros of the function cannot be determined exactly. The first positive zero is close to 1. To find its numerical approzimation we use

  FindRoot[ff[x] == 0, {x, 1},
      AccuracyGoal -> Infinity, PrecisionGoal -> 12,
      MaxIterations -> 1000, WorkingPrecision -> 16]

  Usually we do not need this level of precision, but it is nice to know how to achieve it. Other positive zeros come in pairs around even positive multiples of $\pi.$ Here is the pure function combined with Map[] which finds the first 12 pairs

  {x /. FindRoot[ff[x] == 0, {x, 2 # Pi - .1},
      AccuracyGoal -> Infinity, PrecisionGoal -> 12,
      MaxIterations -> 1000, WorkingPrecision -> 16],
  x /. FindRoot[ff[x] == 0, {x, 2 # Pi + .1},
      AccuracyGoal -> Infinity, PrecisionGoal -> 12,
      MaxIterations -> 1000, WorkingPrecision -> 16]} & /@ Range[1, 12]

  The same outcome produced with Table[] is

  Table[
   {x /. FindRoot[ff[x] == 0, {x, 2 k Pi - .1},
      AccuracyGoal -> Infinity, PrecisionGoal -> 12,
      MaxIterations -> 1000, WorkingPrecision -> 16],
    x /. FindRoot[ff[x] == 0, {x, 2 k Pi + .1},
      AccuracyGoal -> Infinity, PrecisionGoal -> 12,
      MaxIterations -> 1000, WorkingPrecision -> 16]},
         {k,1, 12}]

• Notice the use of semicolon (; ) in a cell. Its purpose is to separate different commands used in the same cell. Contrast this with the use of comma (, ) which is used to separate variables in a command which takes several variables.
• Also notice the use of colon (: ) in the definition of function. For the meaning of := check SetDelayed in the help file. Basically, if you use :=, then Mathematica will remember the formula and evaluate it only when we use the function later on. I prefer to use := in definitions of functions, in particular complicated ones. Often, like in the simple example above, := and = are interchangeable. However, this is not always the case. Here is one example in which = yields a meaningful function while := does not make sense: Say I want to graph three random lines with y-intercepts and slopes in the interval [0,1]. I can define

  Clear[f1, f2, f3, x];
      f1[x_] = Random[] + x*Random[];
      f2[x_] = Random[] + x*Random[];
      f3[x_] = Random[] + x*Random[];

  Now you are thinking that I defined the same function each time. Try graphing:

  Plot[
      {f1[x],f2[x],f3[x]}, {x,-5,5},
        PlotStyle->{{Thickness[0.007],Red}, {Thickness[0.007],Green}, {Thickness[0.007],Blue}},
        PlotRange->{-5,5}, AspectRatio->Automatic
      ]

  and you will see that these are different functions since in each definition Mathematica assigned different random numbers as slopes and y-intercepts. I am writing all of this to point out that

  Clear[g,x];
      g[x_]:= Random[] + x*Random[];

  does not make sense! With this definition Mathematica selects random numbers as slope and y-intercept whenever you calculate. For example, g[1]. Try evaluating it twice. You get different numbers! Thus, as defined, g[x] is not a function.
• The function sinc is in Mathematica's huge library of functions. Clearly Sinc is not a bijection. However, in the spirit of inverse trigonometric functions one can define ArcSinc[x] by restricting the domain for the independent variable x. This is a nice exercise is using Interpolation[] function in Mathematica.

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Recursively defined functions

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• Functions whose domain is the set of positive integers can be defined recursively. Here is an example. Why I choose this long name Repeat will be clear after you define and explore this function.

  Clear[Repeat, n];
      Repeat[1] = 1;
      Repeat[n_]:= Repeat[n - Repeat[n - 1]] + 1

  But much more complicated functions are common. For example functions that include graphing commands.
• If you try to calculate Repeat[100] you will notice that Mathematica takes quite a long time to calculate this number. Try

  Timing[Repeat[60]]

  On my computer it takes 2.735 seconds to calculate that Repeat[60] = 11. The reason for this is that as defined above the function Repeat[] does not remember any values that it calculates. We can make Mathematica remember all the function values it finds by changing the syntax in the definition:

  Clear[Repeat, n];
      Repeat[1] = 1;
      Repeat[n_]:= Repeat[n] = Repeat[n - Repeat[n - 1]] + 1

  You will easily notice that this definition is much faster.
• Test what this function does using

  Table[Repeat[k], {k, 1, 106} ]

  or by plotting its graph (which consists of points with integer coordinates)

  Show[
     Graphics[{
      {PointSize[0.015], Table[Point[{k, Repeat[k]}], {k, 1, 106}]}
            }],
     PlotRange -> {{0, 107}, {0, 15.5}},
     Frame -> True, ImageSize -> 400
       ]

  But sometimes we want to control more details of the plot:

  Show[
    Graphics[{ (* the start of the graphics, it is a list *)
      { (* the grid lines are in this list *)
      Thickness[0.003], GrayLevel[0.8],
      Table[Line[{{0, j}, {107, j}}], {j, 1, 15}],
      Table[Line[{{j(j + 1)/2, 0}, {j(j + 1)/2, 16}}], {j, 1, 15}]
      }, (* the next list are the points on the graph*)
      {PointSize[0.015], Blue, Table[Point[{k, Repeat[k]}], {k, 1, 107}]}
           }], (* the end of the graphics *)
     PlotRange -> {{0, 107}, {0, 15.5}},
     AspectRatio -> 1/2,
     Frame -> True,
     FrameTicks -> {Table[j(j + 1)/2, {j, 1, 15}], Range[1, 15], {}, Range[1, 15]},
     ImageSize -> 400
       ]

  Please notice the indentation of the parts of the command. That helps me keep me identify the code for each part of the picture. You can copy-and-paste (^C then ^V) this gray box into your Mathematica notebook.
• I used the Repeat[] function above as an illustration for a recursive definition. But there are explicit formulas that define the same function. Such formulas are called "closed form expressions" for a recursive sequence. Here are few closed form expressions for Repeat[]:

  Clear[Rf1, Rf2, Rf3, n];
      Rf1[n_]:= Round[Sqrt[2*n]];
      Rf2[n_]:= Floor[(1/2) + Sqrt[2*n]];
      Rf3[n_]:= Ceiling[(-1/2) + Sqrt[2*n]];

  The following table is a quite convincing evidence of the above claim:

  Table[
         {k, Repeat[k], Rf1[k], Rf2[k], Rf3[k]},
                                                {k, 1, 100}
       ]//TableForm

• I just want to mention that the determining a closed form expression for a recursive sequence is a hard problem. For example, the creator of Mathematica Stephen Wolfram in 2002 introduced the following slight modification of the Repeat[] sequence:

  Clear[W, n];
      W[1] = 1; W[2] = 1;
      W[n_]:= W[n] = W[n - W[n - 1]] + 2

  I believe that I am the first one to come up with a closed form expression for this sequence. On February 10, 2011 I found the following closed form expression for the above sequence

  Clear[Wf1, n];
      Wf1[n_]:= 2*Floor[Sqrt[n]]-(-1)^(n+Ceiling[Sqrt[n]])

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