Lecture 3: SageMath as a Calculator – getting Help

In this lecture we show how to use the SageMath notebook as a calculator and explore the extensive help facilities.

If we compute with arbitrary multiprecision, then we must raise the precision of the field to compute the rounding errors correctly. We illustrate this with the computation of a numerical approximation to \(\pi\), accurate up to 30 decimal places. We explore the help system of SageMath, priming the continued fractions of the next lecture.

Getting Started

We run the notebook in multicell mode. In the command line interface, the commands are executed in sequence, in the notebook interface, we can execute commands out of order.

We enter mathematical expressions in a cell. The expressions are executed by clicking on the evaluate button which appears below each cell. For example:

34^34

With the underscore we get the result from the previous computation. Note that previous refers to time, not location. To experience this, we add in the next cell the underscore, and then in the following cell:

2*3

When we evaluate we see the result 6 appear and when we evaluate the cell with the underscore that is located above the cell 2*3, then we will see the result change from the value of 34^34 into 6. Consider the next cell with content

pi
pi - _

Repeatedly clicking on the evaluate button will give different results each time. Can you explain the differences?

At the command line interface: (but not in the notebook interface) we can use double and triple underscores:

sage: 2
2
sage: 3
3
sage: 4
4
sage: ___
2
sage: ___
3
sage: ___
4
sage: __
3
sage: _
3

Consider the computation of the difference between pi and a 30 digit numerical approximation for pi. The shortest way to obtain this is via pi.n(digits=30). Another way is via numerical_approx as illustrated below:

numerical_approx(pi,digits=30)

But often we want to compute with a precision of 30 decimal places. Then we compute with a RealField of prescribed precision. For a 30-digit approximation of pi we then type in a cell:

R = RealField(30*log(10,2))
pi30 = R(pi)
strpi30 = str(pi30)
print('%d-digit approximation for pi : %s' % (len(strpi30),strpi30))

With log(10,2) we compute the binary logarithm of 10, because the argument of RealField is expressed in bits. After the creation of R, we convert the symbol for the mathematical constant pi into a 30-digit approximation for pi. This approximation is stored in the variable pi30. The conversion to the string representation str(pi30) is used to check that indeed we do have 30 decimal places, as in confirmed by the output produced by the cell.

As simple computation of the difference between pi and pi30 can be done as

R(pi - pi30)

which returns 0.00000000000000000000000000000

The problem with the previous computation is that the precision of the RealField was not large enough. To make the difference with 40 decimal places:

RealField(40*log(10,2))(pi-pi30)

yields 1.69568553528388823010177990083205496387e-31. Thus we verified that pi30 has the desired accuracy of 30 decimal places.

Next we illustrate the difference between precision and accuracy:

pi40 = RealField(40*log(10,2))(pi30)

we have made a variable pi40 with a working precision of 40 decimal places. But because it takes the value of pi30, the accuracy of pi40 is no more than 30 decimal places.

Getting Help

To see the documentation for a specific command, in a cell we can type

help(RealField)

To look for a specific topic, we can use the search_doc as illustrated below:

search_doc("numerical approximation")

Then the user gets redirected to the extensive online documentation, shown in Fig. 5 below.

_images/figsearchdoc.png

Fig. 5 The documentation in SageMath consists of tutorials, the reference manual, constructions (how do I construct .. in Sage?), and PREP tutorials aimed at undergraduate students.

The list in Fig. 5 was produced with version 8.6 of SageMath. As the documentation has grown, the output with current versions is much larger.

The Packages in SageMath

There are many packages in SageMath where we can find pi:

print(type(pi))
print(type(math.pi))
import sympy
print(type(sympy.pi))
import numpy
print(type(numpy.pi))

The second and fourth type is the ordinary float whereas the first and third type are symbolic.

To work with the pi as defined by sympy:

from sympy import pi as sympypi
print(sin(sympypi))
print(sympypi.evalf(30))

The first number we see is 0 followed by a 30-digit approximation for pi.

Assignments

In assignments where you are asked to explain, please formulate your answers with complete sentences.

  1. Explain the differences between pi and Pi in SageMath. Illustrate the differences with the appropriate commands.

  2. Verify that SageMath knows that the exponential of the natural logarithm of 2 equals 2.

  3. The mathematical constant e is just as pi a transcendental number. Show that SageMath knows the constant e by taking its natural logarithm. Calculate a 30-digit approximation for e. Verify the accuracy of this approximation by calculating its difference with the exact value.

  4. Consider the following sequence of commands:

    R = RealField(30*log(10,2))
    print(tan(R(pi/2)))
    print(R(tan(pi/2)))
    

    Why do these two commands give different answers? Which of the two commands gives the correct answer? Also compare R(tan(pi)) with tan(R(Pi)) and explain the differences.

  5. The decomposition of 2015 as a product of prime numbers is 5 * 13 * 31. Use the help system of SageMath to find the command to compute the factorization of a natural number.

  6. Explain the difference between 0 and 0.000 and give two good examples of SageMath calculations that result in 0 and 0.000.