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   A Beastly Number
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   Author  Topic: A Beastly Number  (Read 1798 times)
ThudnBlunder
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A Beastly Number  
« on: Nov 26th, 2008, 9:05am »
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a) Find the first 6 digits of (10666)!
 
b) How many trailing zeros does the above number have?
« Last Edit: Nov 26th, 2008, 1:12pm by ThudnBlunder » IP Logged

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Barukh
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Re: A Beastly Number  
« Reply #1 on: Dec 2nd, 2008, 10:32am »
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For an easier part b), I get: 2664(5667 - 1)
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Barukh
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Re: A Beastly Number  
« Reply #2 on: Dec 3rd, 2008, 12:31am »
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According to the following article, solving part a) may require calculation of a certain logarithm to a precision of more than 670 decimal digits!
 
 Shocked
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ThudnBlunder
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Re: A Beastly Number  
« Reply #3 on: Dec 4th, 2008, 6:56pm »
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on Dec 2nd, 2008, 10:32am, Barukh wrote:
For an easier part b), I get: 2664(5667 - 1)

If we take n/4 as an estimate for the number of trailing zeros, we get 2.5*10665
Your number is 5 times this.
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Barukh
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Re: A Beastly Number  
« Reply #4 on: Dec 5th, 2008, 12:15pm »
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on Dec 4th, 2008, 6:56pm, ThudanBlunder wrote:

Your number is 5 times this.

Yes, I should've written 5666 instead. But now I realized the answer is incorrect anyway (doesn't take into account fractions).  
 
To write the answer in a "compact form" may be as difficult as part a) then...
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SMQ
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Re: A Beastly Number  
« Reply #5 on: Dec 5th, 2008, 12:32pm »
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2) 10666/5n
n=1
Where in practice the upper bound can be reduced to 666 log 10 / log 5 = 952

Edit:
 

= 26665666 / 4 +
 
 
286

n=1
 
 
2666/5n
 
Edit2: all of which I think I can safely assume anyone hanging around in Putnam already knew (and is clearly the driver for Barukh's answer), but since this thread has only seen a little action I thought I'd put it out there anyway. Wink
 
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« Last Edit: Dec 5th, 2008, 1:19pm by SMQ » IP Logged

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Barukh
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Re: A Beastly Number  
« Reply #6 on: Dec 6th, 2008, 9:52am »
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SMQ, you are right. My answer is wrong, since it doesn't take into account the second term in your formula - the sum which is challenging to evaluate.
 
After working it out, I get the following answer to b): 26645666 - 143.  Roll Eyes
 
I will supply details later, after I find the answer to the first question.
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Eigenray
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Re: A Beastly Number  
« Reply #7 on: Dec 6th, 2008, 12:25pm »
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on Dec 6th, 2008, 9:52am, Barukh wrote:
After working it out, I get the following answer to b): 26645666 - 143.  Roll Eyes

So is there a clever way to compute the sum of the digits of 2666 = 34004...233245?
 
Actually, the approximation 333*log5(2) is pretty good here.
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Barukh
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Re: A Beastly Number   Zeros.PNG
« Reply #8 on: Dec 7th, 2008, 6:27am »
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on Dec 6th, 2008, 12:25pm, Eigenray wrote:

So is there a clever way to compute the sum of the digits of 2666 = 34004...233245?

I don't know. I used high-precision software (MPFR) to compute the number (I still want to get a confirmation it's correct).
 
Quote:
Actually, the approximation 333*log5(2) is pretty good here.

Yes, but it may be quite inaccurate for other exponents. In the attached graph, I plotted the discrepancies between your approximation and actual number for all cases 10n with 10 < n < 2000.
 
Again, if my calculations are correct.
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Barukh
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Re: A Beastly Number  
« Reply #9 on: Dec 7th, 2008, 11:21pm »
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I get the following answer to part a):
 
13407273847...
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Eigenray
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Re: A Beastly Number  
« Reply #10 on: Dec 8th, 2008, 7:57am »
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on Dec 7th, 2008, 11:21pm, Barukh wrote:
I get the following answer to part a):
 
13407273847...

Are you sure it's not 134072738469787?  But the first 6 digits are correct Smiley
 
And the 143 is correct, as the following short (but rather inefficient) program shows:
Code:

#include<stdio.h>
int A[300];
int main() {
  int n,i,c;
  A[0]=1;
  for(n=0;n<666;n++) for(c=i=0;i<300;i++) {
    c+=2*A[i];
    A[i]=c%5;
    c/=5;
  }
  for(c=i=0;i<300;i++) c+=A[i];
  printf("%i\n",c/4);
}
« Last Edit: Dec 8th, 2008, 8:21am by Eigenray » IP Logged
Barukh
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Re: A Beastly Number  
« Reply #11 on: Dec 8th, 2008, 10:19am »
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on Dec 8th, 2008, 7:57am, Eigenray wrote:

Are you sure it's not 134072738469787?

Hmm... I did calculate the logarithm with very high precision, so at least 15 digits should be accurate. Then, I did exponentiation with a double precision. Could it be I lost 5 digits of accuracy there?  Huh
 
What's your method?
 
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Re: A Beastly Number  
« Reply #12 on: Dec 8th, 2008, 12:44pm »
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Code:

In[1]:= u=1-FractionalPart[ N[10^666 Log[10,E], 686] ]
Out[1]= 0.72825054586682085824
In[2]:= v=N[Log[10,Sqrt[2 Pi]], 20]
Out[2]= 0.39908993417905752478
In[3]:= 10^(u+v)
Out[3]= 13.407273846978712508

What do you get?
 
Edit: And here it is with MPFR:
Code:
#include<stdlib.h>
#include<stdio.h>
#include<gmp.h>
#include<mpfr.h>
 
int main() {
  mpfr_t u,v;
  mpfr_set_default_prec(10000);
 
  mpfr_init_set_ui(v,10,GMP_RNDN);
  mpfr_pow_ui(v,v,666,GMP_RNDN); //v=10^666
 
  mpfr_init_set_ui(u,1,GMP_RNDN);
  mpfr_exp(u,u,GMP_RNDN);
  mpfr_log10(u,u,GMP_RNDN);  //u=log_10 e
  mpfr_mul(u,u,v,GMP_RNDN);
  mpfr_frac(u,u,GMP_RNDN);  //u=frac[10^666 log_10 e]
   
  mpfr_set_si(v,-1,GMP_RNDN);
  mpfr_acos(v,v,GMP_RNDN);
  mpfr_mul_ui(v,v,2,GMP_RNDN);  //v=2pi
  mpfr_log10(v,v,GMP_RNDN);
  mpfr_div_ui(v,v,2,GMP_RNDN);  //v=log(sqrt(2pi))
 
  mpfr_sub(v,v,u,GMP_RNDN);
  mpfr_ui_pow(v,10,v,GMP_RNDN);  //10^(v-u)
  mpfr_out_str(stdout,10,20,v,GMP_RNDN);
  printf("\n");
}

Gives 1.3407273846978712508
 
(I already had GMP but apparently it doesn't do logs.  So I downloaded the MPFR sources and started ./configure; make.  Then I realized I could install it through Cygwin, and wrote the above before it finished compiling.)
« Last Edit: Dec 8th, 2008, 2:00pm by Eigenray » IP Logged
ThudnBlunder
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Re: A Beastly Number  
« Reply #13 on: Dec 9th, 2008, 4:03pm »
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My source for this problem got yet a different answer, 1.340727397...
« Last Edit: Dec 9th, 2008, 4:06pm by ThudnBlunder » IP Logged

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Eigenray
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Re: A Beastly Number  
« Reply #14 on: Dec 9th, 2008, 5:32pm »
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The funny thing is that he did compute the fractional part of 10666/log(10) correctly to 14 digits.  But he decided to round it to 8 digits before exponentiating, and then claim 10 digits of accuracy in the result:
Quote:
{2} x 10-0.27174945 = 1.340727397,

which is silly.
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