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Q: Elementary Number Theory Identity ( No Answer,   3 Comments )
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Subject: Elementary Number Theory Identity
Category: Science > Math
Asked by: mathoverdose-ga
List Price: $50.00
Posted: 23 Feb 2006 14:08 PST
Expires: 25 Mar 2006 14:08 PST
Question ID: 700085
I've been working on the following prime number related stuff over the
last year and a half.  I've been unable to find reference to ANY of it
in any of the number theory books / papers I've read through.  So my
question is, is any of the following known (and if so, where should I
look to read more about it), and is any of it noteworthy?

Section 1: Prime Power Function
-------------------------------

For any positive constant integer N, where n1, n2, ... nx are natural
number variables greater than or equal to 2,

        #{ n1                     = N : nx >= 2 }
- 1/2 * #{ n1 * n2                = N : nx >= 2 }
+ 1/3 * #{ n1 * n2 * n3           = N : nx >= 2 }
- 1/4 * #{ n1 * n2 * n3 * n4      = N : nx >= 2 }
+ 1/5 * #{ n1 * n2 * n3 * n4 * n5 = N : nx >= 2 }
- ...

which is to say, the count of ways that N can be expressed as an
integer greater than 1, minus half the count of ways that N can be
expressed as two integers multiplied together, each greater than 1,
plus a third of the count of ways N can be expressed as three integers
each greater than 1 multiplied together, and so on is equal to the
following:

0         if N is composite
1         if N is prime
1 / power if N is a prime power

So, for example,

245
---

+ 1 / 1 * #{ (245)                          } = +1
- 1 / 2 * #{ (5*49), (7*35), (35*7), (49*5) } = -2
+ 1 / 3 * #{ (5*7*7), (7*5*7), (7*7*5)      } = +1
                                                --
                                                 0
                                             
Thus, 245 is composite.

81
--
+ 1 / 1 * #{ (81)                      } = +1
- 1 / 2 * #{ (3*27), (9*9), (27*3)     } = -3/2
+ 1 / 3 * #{ (3*3*9), (3*9*3), (9*3*3) } = +1
- 1 / 4 * #{ (3*3*3*3)                 } = -1/4
                                           ----
                                           1/4
                                           
Thus, 81 factors to a prime to the 4th power.



Section 2: Prime Powers Counting Function
-----------------------------------------

The identity from Section 1 requires, generally, factoring N before we
can find the number of solutions for each count.  If, however, we sum
this identity for all values from 2 to N, we find that

        #{ n1                     <= N : nx >= 2 }
- 1/2 * #{ n1 * n2                <= N : nx >= 2 }
+ 1/3 * #{ n1 * n2 * n3           <= N : nx >= 2 }
- 1/4 * #{ n1 * n2 * n3 * n4      <= N : nx >= 2 }
+ 1/5 * #{ n1 * n2 * n3 * n4 * n5 <= N : nx >= 2 }
- ...

is equal to the count of primes <= N plus one half the count of primes
less than or equal to N^(1/2) plus one third the count of primes less
than or equal to N^(1/3) and so on, or, in other words,

        #{ n1 <= N      : n1 prime }
+ 1/2 * #{ n1 <= N^(1/2): n1 prime }
+ 1/3 * #{ n1 <= N^(1/3): n1 prime }
+ 1/4 * #{ n1 <= N^(1/4): n1 prime }
+ 1/5 * #{ n1 <= N^(1/5): n1 prime }
+ ...

Counting these lattice volumes is not trivial, but it can be done
without resorting to factoring.



Section 3: Prime Counting Function
----------------------------------

The prime powers coutning function from Section 2 yields precisely the
same values as Riemann's similar function.  Thus, we can use the same
techniques, employing a mobius inversion to yield the exact count of
primes less than or equal to N.




Section 4: Lattice Counting
---------------------------

For an expression like

#{ n1 * n2 * ... nx <= N: n1, n2, ... nx >= 2 }

any given set of numbers will show up at most x! times.  Additionally,
the smallest value in the set will be at most floor( N^( 1 / x ) ). 
If we count the number of values in the lattice from smallest value to
largest, our process can look, very roughly, something like this:

for( a_1 = 2 to N^(1/x) )
   for( a_2 = a_1 to ( N^(1/(x-1) )/a_1 )
      for( a_3 = a_2 to ( N^(1/(x-2))/(a_1*a_2) )
          ...
          for( a_(x-1) = a_(x-2) to ( N^(1/2)/( a_1*a_2*...*a_(x-2) ) )
             total += b1 + ( N / ( a_1*a_2*...*a_(x-1) ) - 1 ) * b2

where b2 is calculated based on how many times values in the set( a_1,
a_2, ..., a_(x-1) ) are equal to each other.  This technique can be
vastly sped up by using a wheel.

With my current implementation of lattice counting, which is an area
of research I don't know much about, this yields a prime counting
algorithm that works, at least empirically, in O( N ^ 4/5 + epsilon )
time and O( epsilon ) space.  I have this coded and working right now.



Section 5: Continuous Volumes Bounding Lattices
-----------------------------------------------

As before, with n1, n2, ... nx integers >= 2 and N a constant value,

#{ n1 * n2 <= N } is bounded by the continuous (real-variable) curve
x1*x2 = N, x1 >= 1, x2 >= 1.  All valid integer pair solutions to the
first equation are contained entirely by this volume, and only integer
pair solutions from this first equation are contained entirely by this
volume.  The integral for this volume is N log N - N + 1.

#{ n1 * n2 * n3 <= N } is bounded by the continuous curve x1*x2*x3 =
N, where, x1, x2, and x3 are all >= 1.  The integral for this volume
is N/2 * ( log N )^2 - N log N + N - 1.  It has similar properties to
the first equation in terms of the specific lattice points it bounds.

#{ n1 * n2 * n3 * n4 <= N } is bounded by a volume that is, when
integrated, equal to N/6 * ( log N )^3 - N/2 * ( log N )^2 + N log N -
N + 1.

And so on.  All of these volumes have similar properties regarding the
lattice points that they bound.



Section 6: Relationship to the Prime Number Theory
--------------------------------------------------

If we replace the prime power counting function from section 2,

        #{ n1 <= N                     : nx >= 2 }
- 1/2 * #{ n1 * n2 <= N                : nx >= 2 }
+ 1/3 * #{ n1 * n2 * n3 <= N           : nx >= 2 }
- 1/4 * #{ n1 * n2 * n3 * n4 <= N      : nx >= 2 }
+ 1/5 * #{ n1 * n2 * n3 * n4 * n5 <= N : nx >= 2 }
+ ...

with the continuous variable volumes bounding each of these lattices
as approximations, we arrive at the following equation:

        ( - 1 + N )
- 1/2 * (   1 - N + N log N )
+ 1/3 * ( - 1 + N - N log N + N/2 ( log N )^2 )
- 1/4 * (   1 - N + N log N - N/2 ( log N )^2 + N/6 ( log N )^3 )
+ 1/5 * ( - 1 + N - N log N + N/2 ( log N )^2 - N/6 ( log N )^3 + N/24( log N )^4 )
-...

This value is equal the Logarithmic Integral, Li( N ).

It might be possible one could construct an elementary proof of the
prime number theory based off of these relationships, assuming that
all the steps to this point were proven.



Section 7: Relationship to the Zeroes of the Riemann Zeta Function
------------------------------------------------------------------

The equation from Section 2,

        #{ n1 <= N                     : nx >= 2 }
- 1/2 * #{ n1 * n2 <= N                : nx >= 2 }
+ 1/3 * #{ n1 * n2 * n3 <= N           : nx >= 2 }
- 1/4 * #{ n1 * n2 * n3 * n4 <= N      : nx >= 2 }
+ 1/5 * #{ n1 * n2 * n3 * n4 * n5 <= N : nx >= 2 }
- ...

is precisely equal to

        #{ n1 <= N      : n1 prime }
+ 1/2 * #{ n1 <= N^(1/2): n1 prime }
+ 1/3 * #{ n1 <= N^(1/3): n1 prime }
+ 1/4 * #{ n1 <= N^(1/4): n1 prime }
+ 1/5 * #{ n1 <= N^(1/5): n1 prime }
+ ...

which, by Riemann's work, is equal to

Li( N ) - Li( N^( each non-trivial zeta zero ) ) - ln 2 + tiny integral

Given that the formula involving the bounding continuous volumes from
the preceding section equals Li( N ), we can see that

        ( - 1 + N
          - #{ n1 <= N : nx >= 2 } )
- 1/2 * (   1 - N + N log N
          - #{ n1 * n2 <= N : nx >= 2 } )
+ 1/3 * ( - 1 + N - N log N + N/2 ( log N )^2
          - #{ n1 * n2 * n3 <= N : nx >= 2 } )
- 1/4 * (   1 - N + N log N - N/2 ( log N )^2 + N/6 ( log N )^3
          - #{ n1 * n2 * n3 * n4 <= N : nx >= 2 } )
+ 1/5 * ( - 1 + N - N log N + N/2 ( log N )^2 - N/6 ( log N )^3 + N/24( log N )^4
          - #{ n1 * n2 * n3 * n4 * n5 <= N : nx >= 2 })
- ...

which represents volumes of the various bounding curves that aren't
contained by lattice squares/cubes/etc underneath them (and which,
when totaled, is the amount that Li( N ) deviates from pi( N ) +
1/2pi( N^1/2) + 1/3pi( N^1/3) + ... ) is thus equal to

- Li( N^( each non-trivial zeta zero ) ) - ln 2 + tiny integral

What repurcussions, if any, this has is unclear to me.




Section 8: Mu Function
----------------------

For any positive constant integer N, where n1, n2, ... nx are natural
number variables greater than or equal to 2, and where the syntax #{ a
* b <= c } denotes the count of solutions to a * b <= constant c, the
sum

- #{ n1                     = N : nx >= 2 }
+ #{ n1 * n2                = N : nx >= 2 }
- #{ n1 * n2 * n3           = N : nx >= 2 }
+ #{ n1 * n2 * n3 * n4      = N : nx >= 2 }
- #{ n1 * n2 * n3 * n4 * n5 = N : nx >= 2 }
+ ...

(which is to say, the negative value of the count of ways that N can
be expressed as 1 integer greater than 1, plus the count of ways that
N can be expressed as 2 integers multiplied together, each greater
than 1, minus the count of ways N can be expressed as three integers
each greater than 1 multiplied together, and so on)

is equal to the mobius mu function, where

mu( N ) =  0 if N is not squarefree
           1 if N is squarefree and has an even number of factors
          -1 if N is squarefree and has an odd number of factors



Section 9: Mertens Function
---------------------------

The identity for mu above requires, generally, factoring N to find the
number of solutions for each equation.  If, however, we sum this
identity for all values from 2 to N, we find the following:

- #{ n1                     <= N : nx >= 2 }
+ #{ n1 * n2                <= N : nx >= 2 }
- #{ n1 * n2 * n3           <= N : nx >= 2 }
+ #{ n1 * n2 * n3 * n4      <= N : nx >= 2 }
- #{ n1 * n2 * n3 * n4 * n5 <= N : nx >= 2 }
+ ...

Calculating the number of solutions for each of these values, while
not simple, can be done without resorting to any factoring.  These
values are essentially the number of integer lattice values under
various regular rectangular hyperbola volumes.

This identity is a way of expressing the Mertens Function, the sum of
the mobius mu.



Section 10: d_k
---------------

The count of ways a number N can be represented by k integers >= 1
multiplied together, if N in its canonical prime decomposition is
represented by p1^a1 * p2^a2 * ... * px^ax, is equal to

  ( a1 + k - 1 )! / ( a1! * ( k - 1 )! )
* ( a2 + k - 1 )! / ( a2! * ( k - 1 )! )
* ( ... )
* ( ax + k - 1 )! / ( ax! * ( k - 1 )! )



Section 11: d_k_2
-----------------

The count of ways a number N can be represented by k integers >= 2
multiplied together, if N in its canonical prime decomposition is
represented by p1^a1 * p2^a2 * ... * px^ax, is equal to

  binomial( k, 0 ) * d_k    ( N )
- binomial( k, 1 ) * d_(k-1)( N )
+ binomial( k, 2 ) * d_(k-2)( N )
- ...

and so on until the series terminates.




Section 12: Using d_k_x to calculate Mertens and Pi*
----------------------------------------------------

Using the family of d_k_x functions, we can rewrite the previous results as

Mu( N )           = - d_1_2( N ) +       d_2_2( N ) -       d_3_2( N )
+       d_4_2( N ) - ...
Prime Power( N )  =   d_1_2( N ) - 1/2 * d_2_2( N ) + 1/3 * d_3_2( N )
- 1/4 * d_4_2( N ) + ...

Thus

Mertens( N ) = 
- d_1_2( 1 ) +       d_2_2( 1 ) -       d_3_2( 1 ) +       d_4_2( 1 ) - ...
- d_1_2( 2 ) +       d_2_2( 2 ) -       d_3_2( 2 ) +       d_4_2( 2 ) - ...
- d_1_2( 3 ) +       d_2_2( 3 ) -       d_3_2( 3 ) +       d_4_2( 3 ) - ...
- ...
- d_1_2( N ) +       d_2_2( N ) -       d_3_2( N ) +       d_4_2( N ) - ...

and 

pi*( N ) =
  d_1_2( 1 ) - 1/2 * d_2_2( 1 ) + 1/3 * d_3_2( 1 ) - 1/4 * d_4_2( 1 ) + ...
+ d_1_2( 2 ) - 1/2 * d_2_2( 2 ) + 1/3 * d_3_2( 2 ) - 1/4 * d_4_2( 2 ) + ...
+ d_1_2( 3 ) - 1/2 * d_2_2( 3 ) + 1/3 * d_3_2( 3 ) - 1/4 * d_4_2( 3 ) + ...
...
+ d_1_2( N ) - 1/2 * d_2_2( N ) + 1/3 * d_3_2( N ) - 1/4 * d_4_2( N ) + ...

In general, though, while this appears to be true, it is an extremely
slow way to evaluate these functions.
Answer  
There is no answer at this time.

Comments  
Subject: Re: Elementary Number Theory Identity
From: frankcorrao-ga on 23 Feb 2006 14:21 PST
 
You might want to post this to a more specialized forum, like sci.math
or alt.math.moderated.
Subject: Re: Elementary Number Theory Identity
From: mathtalk-ga on 23 Feb 2006 18:11 PST
 
You might be interested in a related topic or branch of number theory
called "multiplicative functions".  A function F defined on the
natural number is said to be multiplicative if F(m*n) = F(m)*F(n)
whenever m and n are coprime (have no common factor greater than 1).

Also your approach to computing some of these values is similar to the
principle of enumeration or "counting" by inclusion/exclusion.


regards, mathtalk-ga
Subject: Re: Elementary Number Theory Identity
From: danputnam-ga on 15 Mar 2006 12:04 PST
 
There are some results in the literature that are closely related to
what you describe in your question. As far as I can tell, some of your
results are novel, and, yes, I think they are quite noteworthy.

I guess should tell you at the outset that I am only an amateur
mathematician, and actually a complete novice in this area.
Consequently, I may be missing something, and you should take my
comments with a grain (or more) of salt.

I think your results are really fascinating, so please feel free to
email me if you have any further questions. I live near the University
of Illinois, where I have access to the library, so perhaps I can help
with xeroxed articles, or whatever.


Sections 1,10, and 11
-------------------------------

There is a considerable body of work on what are called "ordered
factorizations", and the "Factorisatio Numerorum". You can google
these for yourself, but here are some good references:

<http://mathworld.wolfram.com/OrderedFactorization.html>
<http://www.research.att.com/~njas/sequences/A074206>
<http://www.math.wvu.edu/~mays/Papers/Factorizations.pdf>
<http://www.math.wvu.edu/~mays/Papers/apf7.pdf>

In sections 10 and 11, it appears that you have rediscovered a classic
formula of MacMahon that counts factorizations of all lengths. If you
look at MacMahon's formula in the references above and strip off the
outer summation, I believe the result is the function you define in
section 11.

By the way, the formula on the Mathworld page seems to have a sign
error on index j. The two pdf files have the correct formula, and they
also discuss faster ways of counting factorizations.

As far as I know, your observation of section 1 regarding prime powers
versus composite numbers is novel.


Section 2: Prime Powers Counting Function
-----------------------------------------

I originally happened on your post while I was searching for
information on the summatory Von Mangolt function, also called the
Chebyshev function. Like your function, it counts prime powers, but
uses different weights. It is considered to be much more useful than
the prime counting function because it is easier to analyze:

<http://www.math.ucsb.edu/~stopple/explicit.html>


Section 3: Prime Counting Function
----------------------------------
Amen!


Sections 4,5, and 6
----------------------------------
The Dirichlet divisor problem is a famous old problem that studies the
summatory divisor function.

<http://mathworld.wolfram.com/DirichletDivisorProblem.html>

The divisor function is closely related to the the number of
factorizations of n with two factors. However, it also includes both 1
and n, so it is off by 2. Anyway, the object is to estimate the
summatory divisor function by counting lattice points underneath a
hyperbola. You seem to be dealing with a multidimensional version of
this problem. There is a book by E. Kratzel, "Lattice Points" that
considers multidimensional generalizations of this problem. Also, try
googling for the "Piltz divisor problem", which is one name for the
multidimensional problem.

For an elementary treatment of the 2-dimensional case, see the book
"Number Theory", by George Andrews. See also Hardy and Wright.

The approach you outline in section 6 seems like a really good idea.
If successful, I think it would be a lot more important than finding a
fast prime counting algorithm.
----------------------------------

This is all I have at the present time. As I said earlier, please feel
free to email me. If I have anything more, it might be useful if I had
your regular email address.

Best regards,
Dan Putnam

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