Modular Arithmetic and Exponential Towers: Solving the Power Tower Puzzle

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Discussion Overview

The discussion revolves around calculating the remainder of an exponential tower of three thousand 7's when divided by 11. The focus is on modular arithmetic techniques, particularly in the context of large exponentiation and the application of theorems such as Fermat's Little Theorem and Euler's theorem.

Discussion Character

  • Exploratory, Technical explanation, Mathematical reasoning

Main Points Raised

  • One participant introduces the problem of finding the remainder of the exponential tower 7^(7^(7...)) with three thousand 7's modulo 11, suggesting a connection to the Ackermann Function.
  • Another participant proposes a brute force method to evaluate the tower by calculating the sequence of remainders modulo 11, noting the potential for cycling through values.
  • A different participant suggests using Euler's theorem as an alternative approach, indicating that there are efficient methods for large modular exponentiation.
  • One participant mentions applying Fermat's Little Theorem to reduce the last exponent modulo 10, implying that knowing the remainder of a tower of 2999 7's modulo 10 could simplify the problem.
  • A participant expresses gratitude for the suggestions and indicates plans to discuss the problem further in a group meeting, highlighting their inexperience with modular arithmetic.

Areas of Agreement / Disagreement

Participants present multiple approaches to the problem, and while there are suggestions for methods to simplify the calculations, no consensus on a single solution or method emerges.

Contextual Notes

Participants acknowledge the complexity of the problem and the potential challenges in calculating large exponentiations by hand, suggesting that some methods may be easier with computational tools.

Who May Find This Useful

Readers interested in modular arithmetic, large exponentiation techniques, or those looking for problem-solving strategies in mathematical contexts may find this discussion relevant.

phantasmagoriun
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Consider an exponential tower of three thousand 7's.
What is the remainder when you divide the tower by 11?
Note that this notation means 7^(7^7) not (7^7)^7. So it's kinda like the Ackermann Function maybe?
The final answer must be given as a single integer in the range 0-10.


Anyone got any ideas?
 
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You can just do a (sort of) brute force method:
Find 7 mod 11 = a1
Find 7 ^ a1 mod 11 = a2
Find 7 ^ a2 mod 11 = a3
...
Find 7 ^ a(n-1) mod 11 = an where an = a1
Then the top of the exponent tower will cycle through the sequence a1...an-1 as you evaluate it and all you have to do is count correctly so you land on the right element of the sequence.

This method is easy by calculator but harder by hand. There is probably a better way to do this.
 
[tex]\phi(11)=10[/tex]
So, from Fermat's Little Theorem, we could reduce the last exponent modulo 10 without altering the result.

So, if we knew the remainder of a tower of 2999 7's when divided by 10 then we'd only have one exponentiation left, right?
 
Thanks for the help. I'm going to bring this stuff into my group meeting and see what we can do with it. (None of us have seen this type of problem before, nor worked with mod, so hopefully we can figure it out)
 

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