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## #1 2014-06-14 14:26:47

cooljackiec
Member
Registered: 2012-12-13
Posts: 185

### bases

Let b be an integer greater than 2, and let

(the sum contains all valid base b numbers up to 100_b). Compute the number of values of b for which the sum of the squares of the base b digits are less than or equal to 512. I understand what N_b's value is, but i don't know about the values. Maybe trying smaller values of b would work.

[edited for clarity - bobbym]

Prove that from the set

one can choose 2^k numbers so that none of them can be represented as the arithmetic mean of some pair of distinct chosen numbers.

I see you have graph paper.
You must be plotting something

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## #2 2014-06-14 22:05:33

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

hi cooljackiec

Compute the number of values of b for which the sum of the squares of the base b digits

Is there a bit missing at the end of this sentence?

In base b the column headings are 1, b, b^2, b^3, ......

so

therefore

but then I don't know what I'm doing with this.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #3 2014-06-15 02:58:24

cooljackiec
Member
Registered: 2012-12-13
Posts: 185

### Re: bases

the sum of the squares of the base b digits are less than or equal to 512. I understand what N_b's value is, but i don't know about the values. Maybe trying smaller values of b would work.

I see you have graph paper.
You must be plotting something

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## #4 2014-06-15 06:08:00

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

If I've understood this problem correctly, there are very few cases, so you could just use trial to get them all.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #5 2014-06-15 06:44:18

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

But, you need the squares of the digits...

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #6 2014-06-15 06:54:28

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

OK.  I don't understand the question then.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #7 2014-06-15 06:57:33

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: bases

Compute the number of values of b for which the sum of the squares of the base b digits

Is it my eyes or this browser, is something missing from the end of this line?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #8 2014-06-15 07:27:58

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

Hi bobbym

The remainder of the question is in post #3.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #9 2014-06-15 07:29:24

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: bases

Bob already asked that question. I am going to edit post #1.

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #10 2014-06-15 07:38:58

cooljackiec
Member
Registered: 2012-12-13
Posts: 185

### Re: bases

We just have to sum the squares of the digits in \$N_b\$.  But finding the digits ain't viable. I don't have any ideas.

I see you have graph paper.
You must be plotting something

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## #11 2014-06-15 07:48:30

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: bases

Why is finding the digits not viable?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #12 2014-06-15 07:59:25

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

Because you cannot do it in the general case.

Also, I just figured out that Bob posted the expression for the sum. But, that unfortunately tell us nothing about the digits, though.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #13 2014-06-15 08:06:51

bobbym
bumpkin
From: Bumpkinland
Registered: 2009-04-12
Posts: 109,606

### Re: bases

What do you do with

add the digits and square? Treat the digts as base 10 number and square? What?

In mathematics, you don't understand things. You just get used to them.
If it ain't broke, fix it until it is.
Always satisfy the Prime Directive of getting the right answer above all else.

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## #14 2014-06-15 08:09:30

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

I have it sorted on paper.  Watch this space, while I make a picture.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #15 2014-06-15 08:16:40

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

hi

This is my interpretation of the problem.

The picture shows the cases b = 2, b = 3 and b = 4
I have listed all the values from 1 to b^2

The highlighting shows a clear pattern.

With b = 2, we have 2 ones in the first and second column.
With b = 3, we have 3 ones and 3 twos .
With b = 4, we have 4 ones, 4 twos and 4 threes.

Generalising and adding one more for the one in the third column we get this formula for the sum of the squares of the digits:

This checks out with answers 5, 31 and 113 respectively.

Bob

Last edited by bob bundy (2014-06-15 08:19:56)

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #16 2014-06-15 09:55:55

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

Hi Bob

I'm not sure what you did, so I think it would be the easiest to explain how you got 5 for b=2. I am getting 2.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #17 2014-06-15 10:09:35

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

hi Stefy,

the sum of the squares of the base b digits

Next one:

Next one:

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #18 2014-06-15 10:23:27

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

But, I think we are supposed to find the sum of the squares of the digits of N_b.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #19 2014-06-15 19:02:33

bob bundy
Registered: 2010-06-20
Posts: 8,082

### Re: bases

You may be right.  The problem is difficult to understand.

Bob

Children are not defined by school ...........The Fonz
You cannot teach a man anything;  you can only help him find it within himself..........Galileo Galilei

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## #20 2014-06-15 19:49:01

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

I agree. We should wait for OP to verify which interpretation of the problem is correct.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #21 2014-06-16 02:58:22

cooljackiec
Member
Registered: 2012-12-13
Posts: 185

### Re: bases

Ok, we have N_b. Let's say N_b is in form of

. We want

I see you have graph paper.
You must be plotting something

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## #22 2014-06-16 03:48:00

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

cooljackiec wrote:

Ok, we have N_b. Let's say N_b is in form of

. We want

Hm, unfortunately, just as I thought. This will be harder.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #23 2014-06-16 05:32:27

cooljackiec
Member
Registered: 2012-12-13
Posts: 185

### Re: bases

Do you have any ideas for the 2nd question?

I see you have graph paper.
You must be plotting something

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## #24 2014-06-16 10:20:16

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

Hi cooljackiec

I found that these numbers satisfy Q2.

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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## #25 2014-06-16 11:03:53

anonimnystefy
Real Member
From: Harlan's World
Registered: 2011-05-23
Posts: 16,037

### Re: bases

anonimnystefy wrote:

Hi cooljackiec

I found that these numbers satisfy Q2.

I have been able to prove that fact, unless I make a mistake somewhere. I will post my proof, if needed.

Also, the formula for Q1. is
, I think, so all bases up to 32 have the property.

Last edited by anonimnystefy (2014-06-16 11:20:47)

Here lies the reader who will never open this book. He is forever dead.
Taking a new step, uttering a new word, is what people fear most. ― Fyodor Dostoyevsky, Crime and Punishment
The knowledge of some things as a function of age is a delta function.

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