# Working in base 12...

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• Jul 28th 2006, 03:23 PM
Bartimaeus
Working in base 12...
To work in base 12, we must invent new symbols for digits 10 and 11. Use t for ten and e for eleven. For example, 2x5=t 12
a Write out 3, 5 and 8 times table in base 12, up to e times.
b In the times tables in a, some have answers with the last digit 0. Which tables in base 10 [up to 9 times] have answers with the last digit 0?
c Explain why some tables have answers with last digit 0 and others do not, in base 10 [up to 9 times] and in base 10 [up to e times]
d Which tables in base 12 [up to e times] will have no answers with last digit 0?
• Jul 28th 2006, 07:05 PM
Soroban
Hello, Bartimaeus!

I assume you are somewhat familiar with base-12 notation.

Quote:

To work in base-12, we must invent new symbols for digits 10 and 11.
Use $t$ for ten and $e$ for eleven. For example, $2 \times 5 = t_{12}$

(a) Write out 3-, 5- and 8-times table in base-12, up to $e$-times.

Code:

  x |  1 |  2 |  3 |  4 |  5 |  6 |  7 |  8 |  9 |  t |  e |  -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- +   3 |  3 |  6 |  9 | 10 | 13 | 16 | 19 | 20 | 23 | 26 | 29 |  -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- +   5 |  5 |  t | 13 | 18 | 21 | 26 | 2e | 34 | 39 | 42 | 47 |  -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- +   8 |  8 | 14 | 20 | 28 | 34 | 40 | 48 | 54 | 60 | 68 | 74 |  -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- + -- +
Quote:

(b) In the times tables in (a), some have answers with the last digit 0.
Which tables in base-10 [up to 9-times] have answers with the last digit 0?

In base-10, the rows that have answers ending in 0 are:
. . 2-times, 4-times, 6-times, 8-times, and 5-times.

Quote:

(c) Explain why some tables have answers with last digit 0 and others do not
in base-10 [up to 9-times] and in base-12 [up to e times]

For base-ten: $10 = 2\cdot5$
Any row which shares a common factor with 10 has answers ending in 0.
Any row which is relative prime to 10 has no answers ending in 0.

For base=twelve: $12 = 2^2\cdot3$
Any row which shares a common factor with 12 has answers ending is 0.
. . They are: 2-times, 3-times, 4-times, 6-times, 8-times, 9-times, and $t$-times

Quote:

(d) Which tables in base-12 [up to $e$-times] will have no answers with last digit 0?

Any row which is relatively prime to 12 has no answers ending in 0.
. . They are: 5-times, 7-times, and $e$-times.

• Jul 29th 2006, 12:11 AM
Bartimaeus
Thankyou Soroban. You are a Super member.
I am a little familiar with writing in bases, but never in any higher than base ten. Is there, I suppose you could call it, formula, table or a trick in working in converting on base into another?
• Jul 29th 2006, 09:23 AM
Soroban
Hello again, Bartimaeus!

There is a procedure for changing a base-ten number to another base.

Example: write $389$ in base-5.

Step 1: Divide $389$ by $5$ and note the remainder:
. . . . . . $389 \div 5\:=\:77\quad rem.\boxed{4}$

Step 2: Divide the quotient by 5 and note the remainder:
. . . . . . . $77 \div 5 \,=\,15\quad rem.\boxed{2}$

Repeat Step 2 until a zero quotient each reached.
. . . . . . . $15 \div 5 \,=\,3\quad rem.\boxed{0}$
. . . . . . . . $3 \div 5\,=\,0\quad rem.\boxed{3}\;\;\uparrow$

Now read up the remainders: . $3024_5$

The division can be written like this:
Code:

    5 ) 3 8 9       -------       5 ) 7 7  4         -----       5 ) 1 5  2         -----         5 ) 3  0           ---             0  3 ↑
• Jul 29th 2006, 10:05 AM
galactus
Now, suppose you wanted to change this base 5 to base 10.

$3024_{5}$

Starting from the far right multiply each digit by successive powers of 5(starting with 0):

$4*5^{0}=4$
$2*5^{1}=10$
$0*5^{2}=0$
$3*5^{3}=375$

• Jul 29th 2006, 02:34 PM
Soroban
Nice explanation, Cody!

But has anyone ever seen this method?
[The explanation/derivation is long, but the method is very fast.]

Example: .Evaluate $f(x) = 2x^3 - 5x^2 + 3x - 1$ at $x = 3$

We have: . $f(x)\:=\:2x^3 - 5x^2 + 2x - 1$

Factor $x$ from the first three terms: . $f(x)\:=\:(2x^2 - 5x + 3)\cdot x - 1$

Factor $x$ from the first two terms: . $f(x)\:=\:([2x-5]\cdot x + 3)\cdot x - 1$

Then: . $f(3)\:=\:([2\cdot3 - 5]\cdot3 + 3)\cdot3 - 1$

Now "read" the steps we will take: . $\begin{array}{cccccc}\text{2 times 3}\\ \text{minus 5} \\ \text{times 3} \\ \text{plus 3} \\ \text{times 3} \\ \text{minus 1}\end{array}$ . $\begin{array}{cccccc}6\\ 1\\3\\6\\18\\\boxed{17}\end{array}$

We could have done this mentally ... no squaring or cubing involved.

But if all that factoring is required, it's not much of a shortcut, is it?

Look again at the factored polynomial: . $f(x)\:=\:\left([2x - 5]\[\cdot x + 3\right)\cdot x - 1$

"Drop" the grouping symbols: . $2x - 5\cdot x + 3\cdot x - 1 \;= \;2x - 5x + 3x - 1$

This is the original polynomial with the exponents removed . . .

If we are given: . $g(x)\;=\;3x^4 - 7x^3 + 4x^2 - 9x + 8$
. . we can (mentally) drop the exponents: . $3\cdot x - 7\cdot x + 4\cdot x - 9\cdot x + 8$
. . and we have the factored form (minus the grouping symbols).

For $g(2)$: 3 times 2, minus 7, times 2, plus 4, times 2, minus 9, times 2, plus 8

Important: We must complete each addition/subtraction before the next multiplication.

On our calculator, it would be:
. . $3\;\boxed{\times}\;2\;\boxed{-}\;7\;\boxed{=}\;\;\boxed{\times}\;2\;\boxed{+}\;4 \;\boxed{=}\;\;\boxed{\times}\;2\;\boxed{-}\;9\;\boxed{=}\;\;\boxed{\times}\;2\;\boxed{+}\;8 \;\boxed{=}$

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Back to base-five . . .

Example: Convert $3024_5$ to base-ten.

The value is: . $3\cdot5^3 + 0\cdot5^2 + 2\cdot5 + 4$ .and we have a "polynomial in 5".

So we have: . $3\cdot5 + 0\cdot 5 + 2\cdot 5 + 4\quad\Rightarrow\quad\begin{array}{cccccc}\text{3 times 5} \\ \text{plus 0}\\ \text{times 5}\\ \text{plus 2}\\ \text{times 5}\\ \text{plus 4}\end{array}$ $\begin{array}{cccccc}15\\15\\75\\77\\385\\\boxed{3 89}\end{array}$

~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~

This procedure is even more spectacular in base-two.

Example: Write $101011_2$ in base-ten.

We have: . $1\times2 + 0\times2 + 1\times 2 +0\times2 + 1\times2 + 1$
. . . . . . . . . . . $2\quad\,2\quad\;4\quad5\;\;\;10\;\;\;10\;\;\;20\; \;\;21\;\;42\;\;\:\boxed{43}

$

You have the answer while others are discovering that it begins with $2^5.$

• Jul 29th 2006, 03:46 PM
JakeD
Quote:

Originally Posted by Soroban
But has anyone ever seen this method?

It is Horner's Rule.

And reading that I see "The Horner scheme is often used to convert between different positional numeral systems — in which case x is the base of the number system, and the ai coefficients are the digits of the base-x representation of a given number." And I didn't know that connection. So thanks, Soroban!
• Jul 30th 2006, 06:49 AM
dan
So, I have a question...
Is it possible to write a given number x in a given base y where y is not a integer i.e. write 1220 is base e. The problem I'm having is that to use the traditional method for converting bases with remainders, your remainder is a decimal. Also the practicality seems to run down when you realize that you have e (2.71828182846...) numbers in your system.
Dan
• Jul 30th 2006, 06:58 AM
ThePerfectHacker
Quote:

Originally Posted by dan
So, I have a question...
Is it possible to write a given number x in a given base y where y is not a integer i.e. write 1220 is base e. The problem I'm having is that to use the traditional method for converting bases with remainders, your remainder is a decimal. Also the practicality seems to run down when you realize that you have e (2.71828182846...) numbers in your system.
Dan

No, for example you cannot express,
$\pi = a_0+a_1e+a_2e^2+...+a_ne^n$
Because, they are algebraically independent.
Meaning there is not polynomial with rational coefficients that when evaluavated at $e$ gives $\pi$.
• Jul 30th 2006, 06:58 AM
CaptainBlack
Quote:

Originally Posted by dan
So, I have a question...
Is it possible to write a given number x in a given base y where y is not a integer i.e. write 1220 is base e. The problem I'm having is that to use the traditional method for converting bases with remainders, your remainder is a decimal. Also the practicality seems to run down when you realize that you have e (2.71828182846...) numbers in your system.
Dan

One system: Golden Ratio Base.

I think this is dealt with in general in volume 2 of Don Knuth's TAOCP "SemiNumerical Algorithms", but my copy is at work so I can't check it now.

RonL
• Jul 30th 2006, 07:55 AM
galactus
Quote:

Originally Posted by ThePerfectHacker
No, for example you cannot express,
$\pi = a_0+a_1e+a_2e^2+...+a_ne^n$
Because, they are algebraically independent.
Meaning there is not polynomial with rational coefficients that when evaluavated at $e$ gives $\pi$.

I believe those are called 'transcendental numbers'. Pi and e are classic examples.
• Jul 30th 2006, 08:08 AM
CaptainBlack
Quote:

Originally Posted by ThePerfectHacker
No, for example you cannot express,
$\pi = a_0+a_1e+a_2e^2+...+a_ne^n$
Because, they are algebraically independent.
Meaning there is not polynomial with rational coefficients that when evaluavated at $e$ gives $\pi$.

Have I missed something? Why would you expect the base $e$ representation of $\pi$ to terminate?

RonL
• Jul 30th 2006, 10:15 AM
ThePerfectHacker
Quote:

Originally Posted by CaptainBlack
Have I missed something? Why would you expect the base $e$ representation of $\pi$ to terminate?

RonL

No I said you cannot express it.
• Jul 30th 2006, 10:30 AM
CaptainBlack
Quote:

Originally Posted by ThePerfectHacker
No I said you cannot express it.

To express $\pi$ in base $e$ we would like a representation in the form:

$
\pi = \sum_{r=n_0}^{\infty} a_r\ e^{-r}
$

where $n_0$, may be negative.

In fact does the greedy algorithm not give a construction with all the $a_i$s are either $0,\ 1$ or $2$?

RonL
• Jul 30th 2006, 10:41 AM
CaptainBlack
Quote:

Originally Posted by CaptainBlack
To express $\pi$ in base $e$ we would like a representation in the form:

$
\pi = \sum_{r=n_0}^{\infty} a_r\ e^{-r}
$

where $n_0$, may be negative.

In fact does the greedy algorithm not give a construction with all the $a_i$s are either $0,\ 1$ or $2$?

RonL

It appears that in base $e$ one representation of $\pi$ is:

$
\pi \approx 10.101002020..._{\mbox{base e}}
$

Is this unique? I think so, but haven't checked.

RonL
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