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Agnishom
2013-05-18 03:24:49

Oh Good one! Thanks!

It is easily checked that 677 is prime.

By trying all of 2,3,5,7,11,13,17,19, and 23?

Nehushtan
2013-05-18 00:34:43

It is easily checked that 677 is prime.

PS: In general:

In the above problem:

Agnishom
2013-05-17 22:10:08

Isn't it just the a^2 - b^2 formula?

bobbym
2013-05-17 21:44:30

There are things called aurifeuillian factorizations.

This one could be the basis for many others. But like Aurifeuille who used it for n = 14 in 1871 there is much trial and error.

Agnishom
2013-05-17 21:33:20

How did you come into that formula?

bobbym
2013-05-17 21:00:08

Hi;

Agnishom
2013-05-17 20:39:02

What is the largest prime factor of 5^8 + 2^2?

Nehushtan
2013-04-29 04:00:37

#### Agnishom wrote:

1.For how many odd positive integers n<1000 does the number of positive divisors of n divide n?

As bobbym pointed out, n must be a perfect square. n=1 is one possibility. For the others, it can be easily checked that all odd perfect squares greater than 1 and less than 1000 are have at most two distinct prime factors in their factorization. Thus the possibilities for n>1 are:

where p and q are distinct primes and a, b positive integers.

First case:

The number of positive divisors of n are
– i.e. there are
positive divisors. So the possibilites are
and
. (Not
; that would make n too large.)

Second case:

There are only two such
possible, namely
and
. The number of positive divisors for each number is 9, which does divide each number.

Therefore the answer to your question is: There are 5 odd numbers less than 1000 which are divisible by their number of positive divisors, namely 1, 9, 225, 441, and 625.

bobbym
2013-04-28 19:48:35

That is how I did it. You just square 1,3,5,7,9,11,13,15,17,19,21,23,25,27,29,31 and check.

Agnishom
2013-04-28 19:39:39

Ooh, do we search them manually?

bobbym
2013-04-28 19:29:32

#### bobbym wrote:

The answers are these numbers squared.

1, 3, 15, 21, 25 as given above.

3^2 = 9

Agnishom
2013-04-28 19:27:42

And why not 9 as bob told?

bobbym
2013-04-28 19:01:34

Hi Bob;

The answers are these numbers squared.

1, 3, 15, 21, 25 as given above.

bob bundy
2013-04-28 17:13:35

#### Agnishom wrote:

1.For how many odd positive integers n<1000 does the number of positive divisors of n divide n?

I'm not following this thread at all.

Let's take n = 3

divisors are {1,3} so the number of them is 2.

2 does not divide 3

Take n = 9

divisors are {1,3,9}  That's 3 divisors.  3 divides 9.

I must be misunderstanding something, but I don't know what.

Bob

bobbym
2013-04-28 14:09:25

I am not sure what you are exactly asking so I will answer every possible question.

There is a formula to compute the number of positive divisors of any integer.

those number's divisors have to be odd

Odd numbers have odd divisors.

Even numbers must have one 2 in there prime factorization at least.