# Base Conversion and Modular Arit

# Base Conversion and Modular Arithmetic

In Session 1, we noticed that remainders play a key role
in the repetition or termination of

a decimal . After a long division, if the remainder was zero, the division was
finished and

the decimal terminated. If the remainder was the same number as any previous
remainder,

the pattern would repeat from that point, leading to a repeating decimal.

1. Write the decimal expansion for the base-ten fraction 3/41 . How many
different cycles

will there be in the decimal expansions of the fractions from 1/41 to 40/41?

2. Use long division to write the decimal expansion for the base-ten fraction
1/142857 .

Using division in other bases, it is possible to write “decimal” expansions of
fractions in

other bases.

3. Write the base-ten numbers 1/3 , 1/4 , and 1/7 as “decimals” in base two and
base three.

You may recall from yesterday that we cannot write “3” in base two, so the
divisors

you use will have to be written appropriately.

4. Which fractions in Problem 3 terminated in which bases? Why?

5. Compare the “decimal” expansions of 1/7 in base two, in base three, in base
five, in

base seven, in base ten, in base seventeen (there’s a good reason). When 1/7
repeats,

how long is its repeating pattern in each base? Is there anything unusual going
on

between bases three and five? What about between bases three, ten, and
seventeen?

Look carefully. You might consider using the representation of cycles we used in

Session 1.

We’ve dealt a lot now with converting fractions to decimals, but not decimals to
fractions.

6. Convert the repeating decimal
to a rational number (i.e., a fraction). If you are

not sure how to proceed, check the notes from Session 2.

7. Convert the “eventually repeating” decimal
to a rational number. Think about

what you would multiply the decimal by to create a cancellation.

8. Convert the base-two repeating decimal
to a rational number in base two. Does

this method of solution change from Problem 6 to 8? You might also consider

solving this one as a geometric series.

A slight change of direction now: we are going to start looking at just the
remainders of

calculations performed in bases. For example, in base seven, 3 + 4 = 10. If we
look at only

the remainders when dividing by 7, then the equation reads 3 + 4 = 0.

An arithmetic that uses remainders is referred to as a modular arithmetic. In a
system

governed by remainders when dividing by 7, or modulo 7, the numbers 3, 10, 17,
24, . . . all

become the same number, and “3” would be written to represent this entire class
of

numbers.

We will start with some questions about units digits in base 10, which may seen
unrelated

for a short time.

9. Find the units (last) digit of the calculation 2314 ×
426 + 573 × 234. Do not use a

calculator. Don’t forget about the order of operations !

10. What shortcuts did you use in Problem 9?

11. Find the last digit of (312 × 423 + 57^{2})(28 + 1045 × 68^{3}).
As with Problem 9, try to

do it without a calculator.

12. How can we calculate the units digit of a base-ten number by using the
division

algorithm? Does this explain why units digit arithmetic is a modular arithmetic?

13. Calculate each of these in units digit arithmetic: 3 + 4, 5 + 8, 4 − 3, 2 −
7, 6 × 5,

9 × 9, 0 × 7, 4 ÷ 3.

14. Complete these addition and multiplication tables for units digit
arithmetic.

Unlike ordinary arithmetic, these tables consitute ALL
possible addition and

multiplication problems in this arithmetic.

15. Does order count in addition and multiplication? How can you tell just by
looking

at the structure of the tables?

16. Is it true of this new addition that adding 0 doesn’t change a number?
Explain,

using the table.

The “opposite” or “negative” of a number is the number that you add to it to get
0. In

ordinary arithmetic, the opposite of 4 is −4. In ordinary arithmetic, every
number has an

opposite (what’s the opposite of 0?). In units digits arithmetic, the opposite
of 4 is 6,

because 4 + 6 = 0.

17. Does every number have an opposite in this system? Explain. Give a rule for

determining the opposite of a number, if it has one.

18. Is it true of this new multiplication that multiplying by 1 doesn’t change a
number?

Explain.

The “inverse” or “reciprocal” of a number is the number that you multiply to it
to get 1.

In ordinary arithmetic, the reciprocal of 3 is 1/3 , because
. In the real numbers ,

every number except 0 has a reciprocal. In units digits arithmetic, the
reciprocal of 3 is 7,

because 3 × 7 = 1.

19. Does every number, other than zero, have a reciprocal
in this system? Explain.

20. What’s common among all numbers that have a reciprocal? What’s common among

all numbers that don’t have a reciprocal?

21. In ordinary arithmetic, if the product of two numbers is 0, one of them must
be 0. Is

that true here? Why or why not?

22. Which number in this arithmetic is the “largest”? Explain your answer, using
clear

language.

The system of units digit arithmetic is the system of remainders modulo 10. This
will

usually be referred to either as “mod 10” or Z_{10}. In this course, we
will frequently be

working in different moduli; one purpose is to understand the differences
between working

in moduli and ordinary arithmetic, thus discovering some of the underlying
properties of

“ordinary” arithmetic.

Some examples: in Z_{10}, 3 × 4 = 2, since 3 × 4 = 12 in ordinary
arithmetic, and 2 is the

remainder when dividing 12 by 10. In Z_{7}, 3 × 4 = 5. In Z_{6},
3 × 4 = 0. In Z_{16}, 3 × 4 = 12.

Make a note of the last one: since we are only considering remainders now, we no
longer

have to worry about place value, so there’s no need for C or other letters when
writing

elements of higher mods.

If you’ve made it this far, here are some equations to solve in Z_{10}.
Remember, an equation

is solved by finding all numbers in the set which make the equation true, and Z_{10}
consists

of {0, 1, 2, 3, 4, 5, 6, 7, 8, 9}.

23. Solve x + 6 = 2. Explain how you did it.

24. Solve 3x + 6 = 2. Explain how you did it.

25. Solve 4x + 1 = 5. Explain how you did it.

26. Solve 4x + 1 = 6. Explain how you did it.

27. What makes the solution to Problem 24 different from the solutions to 25 and
26 in

Z_{10}? Write some other equations that look similar, but have very
different sets of

solutions.

28. Repeat Problems 23–26 in Z_{7}, in Z_{11}, in Z_{12}.
How are these systems similar? How

are they different?

29. Build a table for multiplication in Z_{7}, then use it to answer
Problems 15–19.

30. The quadratic equation x^{2} − x = 0 has four solutions, or roots,
in Z_{10}. What are

they? Why is this happening?! Why can’t this happen in ordinary arithmetic?

31. Find all solutions to x^{2} − 6x + 8 = 0 in Z_{5}, in Z_{15},
in Z_{105}. Do the solutions you get

in Z_{5} help you find solutions in Z_{15}?

Two extending questions about base conversion follow.

32. Suppose a number is written as a repeating decimal in base ten. Must it be a

repeating decimal in every other base? If yes, explain why. If no, are there any

particular bases in which it must be a repeating decimal?

33. Use the method of Problems 6–8 to find the value of
What happened? Does

this reveal any assumptions inherent in converting repeating decimals to
fractions?

How do geometric series apply to this problem?

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