# REU Tutorial

## 2902 days ago by admin

Tutorial on Symbolic Expressions and 2D Plotting

This tutorial is mainly about using Sage as a CAS (=Computer Algebra System), i.e., the Mathematica-style aspects of Sage.

# Creating Symbolic Expressions

The "symbolic variable" or indeterminate x is predefined when you startup Sage:

 x x
(x+2)^3
 (x + 2)^3 (x + 2)^3
parent(x)
 Symbolic Ring Symbolic Ring
type(x)
  

Use the var command to define some symbolic variables.  You can separate the variables by commas or spaces in the var command.

reset() x + y
 Traceback (click to the left of this block for traceback) ... NameError: name 'y' is not defined Traceback (most recent call last): File "", line 1, in File "_sage_input_7.py", line 10, in exec compile(u'open("___code___.py","w").write("# -*- coding: utf-8 -*-\\n" + _support_.preparse_worksheet_cell(base64.b64decode("cmVzZXQoKQp4ICsgeQ=="),globals())+"\\n"); execfile(os.path.abspath("___code___.py")) File "", line 1, in File "/tmp/tmphkPXCb/___code___.py", line 3, in exec compile(u'x + y File "", line 1, in NameError: name 'y' is not defined
var('y theta')
 (y, theta) (y, theta)
(x+y+theta)^(2+pi)
 (theta + x + y)^(pi + 2) (theta + x + y)^(pi + 2)
var('a,b,c') expand(a^(b+c))
 a^c*a^b a^c*a^b
show(expand((x+y+theta)^(2+pi)))
f = (x+y+theta)^(2+pi); show(f)
show(f.simplify_full())
print latex(f)
 {\left(\theta + x + y\right)}^{\pi + 2} {\left(\theta + x + y\right)}^{\pi + 2}
sin(2*y)
 sin(2*y) sin(2*y)
var('x y z epsilon')
 (x, y, z, epsilon) (x, y, z, epsilon)
cos(x^3) - y^2*z + epsilon
 -y^2*z + epsilon + cos(x^3) -y^2*z + epsilon + cos(x^3)

NOTE:  If you want symbolic variables to just "magically" spring into existence when you use them, type automatic_names(True).  To turn this off, type automatic_names(False).

automatic_names(True)
expand( (fred + sam + nathan + anything)^3 )
 anything^3 + 3*anything^2*fred + 3*anything^2*nathan + 3*anything^2*sam + 3*anything*fred^2 + 6*anything*fred*nathan + 6*anything*fred*sam + 3*anything*nathan^2 + 6*anything*nathan*sam + 3*anything*sam^2 + fred^3 + 3*fred^2*nathan + 3*fred^2*sam + 3*fred*nathan^2 + 6*fred*nathan*sam + 3*fred*sam^2 + nathan^3 + 3*nathan^2*sam + 3*nathan*sam^2 + sam^3 anything^3 + 3*anything^2*fred + 3*anything^2*nathan + 3*anything^2*sam + 3*anything*fred^2 + 6*anything*fred*nathan + 6*anything*fred*sam + 3*anything*nathan^2 + 6*anything*nathan*sam + 3*anything*sam^2 + fred^3 + 3*fred^2*nathan + 3*fred^2*sam + 3*fred*nathan^2 + 6*fred*nathan*sam + 3*fred*sam^2 + nathan^3 + 3*nathan^2*sam + 3*nathan*sam^2 + sam^3

Using automatic_names is usually a bad idea, since it can easily lead to subtle bugs:

theta^3 - x + y + thetta # "thetta" -- a typo!
 theta^3 + thetta - x + y theta^3 + thetta - x + y
automatic_names(False)
x + y + askdf
 Traceback (click to the left of this block for traceback) ... NameError: name 'askdf' is not defined Traceback (most recent call last): File "", line 1, in File "_sage_input_22.py", line 10, in exec compile(u'open("___code___.py","w").write("# -*- coding: utf-8 -*-\\n" + _support_.preparse_worksheet_cell(base64.b64decode("eCArIHkgKyBhc2tkZg=="),globals())+"\\n"); execfile(os.path.abspath("___code___.py")) File "", line 1, in File "/tmp/tmp6jtL_S/___code___.py", line 2, in exec compile(u'x + y + askdf File "", line 1, in NameError: name 'askdf' is not defined

Using automatic_names also makes it so you don't have to use the foo.method(...) notation:

automatic_names(True)
f = 7*12
f.prime_to_m_part(2)
 21 21
prime_to_m_part(f, 2)
 21 21
automatic_names(False)

Problem: Create the following expressions: $\sin^5(x)\cos^2(x), \qquad \displaystyle \frac{x^3}{x^3 + 1}, \qquad k\cdot P \cdot \left(1 - \frac{P}{K}\right)$.

Tip: that you must put in an asterisk (*) for multiplication.

Tip: You can edit the HTML between cells by double clicking on it.  You can create new HTML areas by shift-clicking on the blue bar that appears just above a compute cell.

Most standard functions are defined in Sage.  They are named lowercase, e.g.,

sin, cos, tan, sec, csc, cot, sinh, cosh, tanh, sech, csch, coth, log, exp, etc.

var('x,y') sin(x) + cos(y) - tan(x/y) + sec(x*csc(y))^5
 sec(x*csc(y))^5 + sin(x) + cos(y) - tan(x/y) sec(x*csc(y))^5 + sin(x) + cos(y) - tan(x/y)

Problem: Construct the symbolic expression $\sin(x^{\cos(y)} + \theta) + \coth(2x) + \log(3x)\cdot \exp(y^3)$.

## Making substitutions

Use the subs method to replace any variables by other variables.

var('x,y') f = sin(x) + cos(y) - x^2 + y
f.subs(x=5)
 y + sin(5) + cos(y) - 25 y + sin(5) + cos(y) - 25
f.subs(x=y, y=x)
 -y^2 + x + sin(y) + cos(x) -y^2 + x + sin(y) + cos(x)
 -x^2 + y + sin(x) + cos(y) -x^2 + y + sin(x) + cos(y)

Problem: Replace $x$ by $\sin(y)-x$ in the expression $x^3 + x y$.

## Expanding Expressions

To expand a symbolic expression with exponents, use the expand method (or function) -- we saw this above:

var('x,y') f = (x+2*y)^3 f
 (x + 2*y)^3 (x + 2*y)^3
f.expand().show() # tip -- using show makes the output nicer
show(f)
latex(f)
 {\left(x + 2 \, y\right)}^{3} {\left(x + 2 \, y\right)}^{3}

Problem: Expand the expression $(2\sin(x) - \cos(y))^5$.

# Symbolic Unit Conversions

As of Sage-4.4, Sage has a massive symbolic unit conversion package, which was written by David Ackerman (a UW undergrad) recently, as  a funded student project after he took Math 480 (the Sage class) last year.

a = units.length.rope
a.convert(units.length.meter)
 762/125*meter 762/125*meter
a = 170*units.mass.pound
a.convert(units.mass.dalton)
 4.64371586715522e28*dalton 4.64371586715522e28*dalton
a = 10 * units.mass.kilogram*units.length.meter / units.time.minute^2; a
 10*kilogram*meter/minute^2 10*kilogram*meter/minute^2
a.convert(units.force.newton)
 1/360*newton 1/360*newton
units.force.newton?
 File: /usr/local/sage-4.4.2/local/lib/python2.6/site-packages/sage/symbolic/units.py Type: Definition: units.force.newton(*args, **kwds) Docstring:  SI derived unit of force. Defined to be kilogram*meter/second^2. File: /usr/local/sage-4.4.2/local/lib/python2.6/site-packages/sage/symbolic/units.py Type: Definition: units.force.newton(*args, **kwds) Docstring:  SI derived unit of force. Defined to be kilogram*meter/second^2.

Problem:  Convert 17 meters/second^2 to miles/minute^2.

#hint d = 17 * units.length.meter / units.time.second^2; d
 17*meter/second^2 17*meter/second^2

Problem: Do a simple conversion that might come up in your own research.

# Creating Symbolic Functions

To create a symbolic function, use the notation f(x,y) = x^3 + y.  A symbolic function is just like a symbolic expression, except you can call it without having to explicitly use subs or name variables and be sure that the order is what you want.

var('x,y') f = y + x^3; f
 x^3 + y x^3 + y
f.subs(y=2,x=3)
 29 29
preparse('f(y,x) = x^3 + y')
 '__tmp__=var("y,x"); f = symbolic_expression(x**Integer(3) + y).function(y,x)' '__tmp__=var("y,x"); f = symbolic_expression(x**Integer(3) + y).function(y,x)'
f(y,x) = x^3 + y f
 (y, x) |--> x^3 + y (y, x) |--> x^3 + y
f(2,3)
 29 29
f(pi,e)
 pi + e^3 pi + e^3

fast_float: this is critical to know about if you're serious about using numerical Python tools that involve repeated function evaluation.   (See also fast_callable for complex and other inputs.)

g = fast_float(f,y,x)
g(2.3, 1.5)
 5.6749999999999998 5.6749999999999998
f(2.3, 1.5)
 5.67500000000000 5.67500000000000
a = float(2.3); b = float(1.5) timeit('g(a,b)')
 625 loops, best of 3: 533 ns per loop 625 loops, best of 3: 533 ns per loop
timeit('f(a,b)')
 625 loops, best of 3: 507 µs per loop 625 loops, best of 3: 507 µs per loop

Problem: Create the functions $x\mapsto x^3 + 1, \qquad (x, y) \mapsto \sin(x) - \cos(y)/y, \qquad (a,x,\theta)\mapsto a x + \theta^2$.

Problem: Evlauate $\sin (x) - \cos(y)/y$ quickly using fast_float.

# 2D Plotting

Use the plot command to plot a function of one variable.  TIP: Type plot(<tab key> to find out much more about the plot command.

var('x') plot(sin(x^2), (x,-10,3))
P = plot(sin(x^2), (x,-10,3)) P.save('image.pdf')
P.save('image.eps')

The plot command has many options:

plot(sin(x^2), (x,-3,3), thickness=5, color='orange', gridlines=True, frame=True, axes=False)

Note for MATLAB Users: You can also use a full emulation of matlab's plotting system via pylab (which is part of the standard matploblib Python library):

from pylab import * clf() t = arange(-3.0, 3.0, 0.05) s = sin(t*t) P = plot(t, s, linewidth=1.0) t = title('MATLAB style plotting') grid(True) savefig('sage.png') reset() # get rid of effect of "from pylab import *"

Problem: If you know MATLAB, try testing some simple 2d plotting.

Here's a the same plot, but you can adjust many of the parameters to the plot command interactively.

var('x') @interact def plot_example(f=sin(x^2),r=range_slider(-5,5,step_size=1/4,default=(-3,3)), color=color_selector(widget='colorpicker'), thickness=(3,(1..10)), adaptive_recursion=(5,(0..10)), adaptive_tolerance=(0.01,(0.001,1)), plot_points=(20,(1..100)), linestyle=['-','--','-.',':'], gridlines=False, fill=False, frame=False, axes=True ): show(plot(f, (x,r[0],r[1]), color=color, thickness=thickness, adaptive_recursion=adaptive_recursion, adaptive_tolerance=adaptive_tolerance, plot_points=plot_points, linestyle=linestyle, fill=fill if fill else None), gridlines=gridlines, frame=frame, axes=axes)

## Click to the left again to hide and once more to show the dynamic interactive window

Problem: Use the above interactive plotter to draw the following plot of $\sin(x^2)$.

You can plot many other things, including polygons, parametric plots, polar plots, implicit plots, etc.:

line, polygon, circle, text, polar_plot, parametric_plot, circle, implicit_plot

You superimpose plots using +.

var('x') P = circle((0,0),1) + polar_plot(2 + 2*cos(x), (x, 0, 2*pi), rgbcolor='red')+ plot(sin(x^2),(x,0,4)) show(P, aspect_ratio=1)

You can also do 3D plots, including parametric lines, parametric surfaces, implicit 3d plots, etc.

var('x y') plot3d(sin(pi*(x^2+y^2))/2,(x,-1,1),(y,-1,1), opacity=.7) + octahedron(color='red')

# More Symbolic Calculus: Computing Integrals and Derivatives

You can symbolically integrate or differentiate functions, compute limits, Taylor polynomials, etc.

var('x') integrate(x^2, x)
 1/3*x^3 1/3*x^3
integrate(sin(x)+tan(2*x),x)
 1/2*log(sec(2*x)) - cos(x) 1/2*log(sec(2*x)) - cos(x)
integrate(sin(x)+tan(2*x),x, algorithm='sympy')
 1/4*log(tan(2*x)^2 + 1) - cos(x) 1/4*log(tan(2*x)^2 + 1) - cos(x)
f = sin(x) - cos(y*x) + 1/(x^3+1) g = f.integrate(x) show(g)
bool(g.differentiate(x) == f)
 True True

# Symbolic Taylor Series

f = sin(x^2)-cos(x)/log(x)
f.taylor(x, 1, 3)
 -7/720*(x - 1)^3*(210*sin(1) + 143*cos(1)) - 1/24*(x - 1)^2*(54*sin(1) - 29*cos(1)) + 1/12*(x - 1)*(6*sin(1) + 31*cos(1)) - cos(1)/(x - 1) + 2*sin(1) - 1/2*cos(1) -7/720*(x - 1)^3*(210*sin(1) + 143*cos(1)) - 1/24*(x - 1)^2*(54*sin(1) - 29*cos(1)) + 1/12*(x - 1)*(6*sin(1) + 31*cos(1)) - cos(1)/(x - 1) + 2*sin(1) - 1/2*cos(1)
g = sin(x^2)/x
g.taylor(x, 0, 20)
 1/362880*x^17 - 1/5040*x^13 + 1/120*x^9 - 1/6*x^5 + x 1/362880*x^17 - 1/5040*x^13 + 1/120*x^9 - 1/6*x^5 + x
@interact def ex_taylor(n=(1..10)): h(x) = sin(x)*cos(x) show(h) show(plot(h,-1,4,thickness=2) + plot(h.taylor(x,1,n),-1,4, color='red'), ymin=-1,ymax=1)

# Other Options for Symbolic Calculus

1. Maxima -- a powerful symbolic computer algebra system, which is fully usable from Sage, and included in Sage.  Sometimes you want to do something, e.g., solve some weird differential equation or evaluate a special functions, and you search the web, find how to do it in Maxima, and can then... paste and do the same in Sage via the Maxima interface.

2. SymPy -- a powerful pure Python library for symbolic Calculus, written almost entirely by Physics graduate students.  This can be installed into anything that you can run Python on, and provides a nice range of capabilities.  Sage uses it some, and it is included in Sage.

## Maxima

maxima('sin(%i)')
 %i*sinh(1) %i*sinh(1)
sin(I)
 sin(I) sin(I)
f = log(sin(x)/x) f.taylor(x, 0, 10)
 -1/467775*x^10 - 1/37800*x^8 - 1/2835*x^6 - 1/180*x^4 - 1/6*x^2 -1/467775*x^10 - 1/37800*x^8 - 1/2835*x^6 - 1/180*x^4 - 1/6*x^2
[bernoulli(2*i) for i in range(1,7)] [1/6, -1/30, 1/42, -1/30, 5/66, -691/2730]
f_maxima = maxima(f) f_maxima
 log(sin(x)/x) log(sin(x)/x)
type(f_maxima)
  
parent(f_maxima)
 Maxima Maxima
maxima(f).powerseries(x,0)
 'sum((-1)^i3*2^(2*i3-1)*bern(2*i3)*x^(2*i3)/(i3*(2*i3)!),i3,1,inf) 'sum((-1)^i3*2^(2*i3-1)*bern(2*i3)*x^(2*i3)/(i3*(2*i3)!),i3,1,inf)

## Sympy

reset()
from sympy import var as svar, sin, integrate, pi, S
x = svar('x') integrate(sin(x)*cos(x+1), (x, 0, pi))
 -pi*sin(1)/2 -pi*sin(1)/2

## Mathematica and Maple

The following won't work for you during the tutorial.  It illustrates Sage's powerful interfaces.

mathematica('Integrate[Sin[x]*Cos[x+1],x]')
 -Cos[1 + 2*x]/4 - (x*Sin[1])/2 -Cos[1 + 2*x]/4 - (x*Sin[1])/2
f = mathematica(sin(x)*cos(x+1)); f
 Cos[1 + x]*Sin[x] Cos[1 + x]*Sin[x]
f.Integrate(x)
 -Cos[1 + 2*x]/4 - (x*Sin[1])/2 -Cos[1 + 2*x]/4 - (x*Sin[1])/2

maple('integrate(sin(x)*cos(x+1),x)')
 -1/4*cos(2*x+1)-1/2*sin(1)*x -1/4*cos(2*x+1)-1/2*sin(1)*x