SharedNotes and Tutorials.ipynbOpen in CoCalc

Notes and Tutorials

This page represents a collection of notes and tutorials for common functions, methods, definitions, etc. used in our class. This is not comprehensive and is intended as an aid for the course. For additional help, use the SAGE cheat sheet, SAGE tutorial, or stackoverflow.


Defining functions using SAGE

There are many ways to define functions in SAGE. The two main methods are:

  • defining a function explictly in terms of variables or
  • defining a variable and then using it in a function formula.
# first method
f(x) = x^2+3
show(f)
x  x2+3x \ {\mapsto}\ x^{2} + 3
# second method
y = var('y')
g = sin(y)
show(g)
sin(y)\sin\left(y\right)

Note that the show command simply produces a nicely formated output using LATXE.\LaTeX.

Differentiating functions in SAGE

There are several ways to differentiate a function using SAGE. If there are multiple variables, you will have to explicitly state the variable you are differentiating with respect to.

# as a method
f.diff()
x |--> 2*x
# as a function
derivative(f)
x |--> 2*x
# for multiple variables
h(x,y) = x^2+y
h.diff(y)
(x, y) |--> 1

Solving an equation in SAGE

The function solve has the format: solve(list of desired equations, variables to solve for {comma separated, not a list}).

# single variable example
solve(3*x+1==0,x)
[x == (-1/3)]
# multiple variable example
solve([x==y,x^2+y^2==1],x,y)
[[x == -1/2*sqrt(2), y == -1/2*sqrt(2)], [x == 1/2*sqrt(2), y == 1/2*sqrt(2)]]

The output of the solve command is a list where each element is a solution to the set of equations. Any expression of the form variable == function/formula/number is a symbolic expression object in SAGE. The right hand side of the equation given by this object can be extracted using the rhs method. (See the example below.)

# the sol varible below is the list of solutions
sol = solve([x==y,x^2+y^2==1],x,y)
show(sol)
[[x=122,y=122],[x=122,y=122]]\left[\left[x = -\frac{1}{2} \, \sqrt{2}, y = -\frac{1}{2} \, \sqrt{2}\right], \left[x = \frac{1}{2} \, \sqrt{2}, y = \frac{1}{2} \, \sqrt{2}\right]\right]

Our system of equations has two solutions. We will extract the yy values of the first solution. (Note that you do not need to use all of the code below to accomplish this task. Here we show every intermediate step so as not to confuse.)

# remember that python/SAGE uses zero as the first index
firstSol = sol[0]
show(firstSol)
[x=122,y=122]\left[x = -\frac{1}{2} \, \sqrt{2}, y = -\frac{1}{2} \, \sqrt{2}\right]
yEquation = firstSol[1]
show(yEquation)
y=122y = -\frac{1}{2} \, \sqrt{2}
yEquation.rhs()
-1/2*sqrt(2)
# single line version of the above work
show(sol[0][1].rhs())
122-\frac{1}{2} \, \sqrt{2}

How to compute the sensitivity of yy with respect to xx

The sensitivity of yy with respect to xx is a measure for the percentage change in yy given a percentage change in xx. It is a unitless quantity defined by S(y,x)=dydxxy.S(y,x)=\dfrac{dy}{dx}\cdot\dfrac{x}{y}. If S(y,x)S(y,x) is close to 0, yy is not sensitive with respect to xx. A negative S(y,x)S(y,x) means that yy and xx have an inverse relationship (as xx goes up, yy goes down and vice versa).

# Note that we use X and Y instead of x and y to avoid problems with the definition of x and y in previous lines
Y(X) = X^2+sin(X)
S(X) = Y.diff()*X/Y
# The method n is for numerical evaluations
S(2).n()
1.46002690481524

The above shows that, when X=2X=2, if XX increases by 1%, then YY increases by 1.46%.

Graphing in SAGE

A plot in CoCalc is an object with various attributes. Graphics objects can be added together to produce multiple function plots on a single graph. Plots have many optional arguments that can add color, axes labels, and legends. Note that labels can be written in LATXE.\LaTeX. Later on we will combine other types of plots together (like list_plot) and use the aspect_ratio optional argument to change how plots are displayed. These graphical objects are built on matplotlib, a module commonly used for plotting by data scientists that use Python.

p1 = plot(x^2,(x,-2,2),axes_labels=['$x$','$y$'],legend_label='$x^2$')
p2 = plot(x,(x,-2,2),color='red',legend_label='$x$')
p1+p2

More to come!