- #1
Bashyboy
- 1,421
- 5
Hello everyone,
Here is the problem:
Let ##\displaystyle f_0() = \frac{1}{1-x}##, and ##f_n(x) = f_0(f_{n-1}(x))##, where ##n \in \mathbb{N}##. Evaluate ##f_{2014}(2014)##.
I first sought after a pattern, and then tried to make a conjecture. Here is what I found:
##\displaystyle f_1(x) = f_0(f_(x)) = \frac{1}{1 - \frac{1}{x-1}} = \frac{1-x}{-x} = \frac{(f_0(x))^{-1}}{-x}##
##\displaystyle f_2(x) = f_0(f_1(x)) = \frac{1}{1- \left(\frac{(f_0(x))^{-1}}{-x} \right)} = \frac{x}{x + (f_0(x))^{-1}}##
##\displaystyle f_3(x) = f_0(f_2(x)) = \frac{1}{1 - \left(\frac{x}{x + (f_0(x))^{-1}} \right)} = \frac{ x + (f_0(x))^{-1} }{x + (f_0(x))^{-1} - x } = \frac{x + (f_0(x))^{-1} }{(f_0(x))^{-1}}##
##\displaystyle f_4(x) = f_0(f_3(x)) = \frac{1}{1 - \left(\frac{x + (f_0(x))^{-1} }{(f_0(x))^{-1} } \right)} = \frac{(f_0(x))^{-1}}{-x} = f_1(x)##
##f_5(x) = f_2(x)##
##f_6(x) = f_3(x)##
So, after the functions ##f_1,~f_2,~f_3##, a pattern begins to emerge, and therefore these three functions are the base functions. The conjecture is that, if you begin with the ##j##-th function, the function will not repeat until the ##(j+3)##-th function.
So, I need to figure out how to "reduce" the number 2014 to either 1,2, or 3 by repeatedly subtracting 3 from 2014 until I arrive at 1,2, or 3. Equivalently, I could repeatedly add 3 to 1,2, and 3, until I reached 2014, and then I would know which function ##f_{2014}## is equivalent to. But I am having difficulty figuring out exactly how to do this without actually doing the addition or subtraction. I know that multiplication of integers is defined in terms of addition; it is repeated addition. So, asking how many times I can add 3 to a number until I get 2014 is related to multiplication, which in turn is related to division, because division is defined in terms of multiplication.
I hope you can help!
Here is the problem:
Let ##\displaystyle f_0() = \frac{1}{1-x}##, and ##f_n(x) = f_0(f_{n-1}(x))##, where ##n \in \mathbb{N}##. Evaluate ##f_{2014}(2014)##.
I first sought after a pattern, and then tried to make a conjecture. Here is what I found:
##\displaystyle f_1(x) = f_0(f_(x)) = \frac{1}{1 - \frac{1}{x-1}} = \frac{1-x}{-x} = \frac{(f_0(x))^{-1}}{-x}##
##\displaystyle f_2(x) = f_0(f_1(x)) = \frac{1}{1- \left(\frac{(f_0(x))^{-1}}{-x} \right)} = \frac{x}{x + (f_0(x))^{-1}}##
##\displaystyle f_3(x) = f_0(f_2(x)) = \frac{1}{1 - \left(\frac{x}{x + (f_0(x))^{-1}} \right)} = \frac{ x + (f_0(x))^{-1} }{x + (f_0(x))^{-1} - x } = \frac{x + (f_0(x))^{-1} }{(f_0(x))^{-1}}##
##\displaystyle f_4(x) = f_0(f_3(x)) = \frac{1}{1 - \left(\frac{x + (f_0(x))^{-1} }{(f_0(x))^{-1} } \right)} = \frac{(f_0(x))^{-1}}{-x} = f_1(x)##
##f_5(x) = f_2(x)##
##f_6(x) = f_3(x)##
So, after the functions ##f_1,~f_2,~f_3##, a pattern begins to emerge, and therefore these three functions are the base functions. The conjecture is that, if you begin with the ##j##-th function, the function will not repeat until the ##(j+3)##-th function.
So, I need to figure out how to "reduce" the number 2014 to either 1,2, or 3 by repeatedly subtracting 3 from 2014 until I arrive at 1,2, or 3. Equivalently, I could repeatedly add 3 to 1,2, and 3, until I reached 2014, and then I would know which function ##f_{2014}## is equivalent to. But I am having difficulty figuring out exactly how to do this without actually doing the addition or subtraction. I know that multiplication of integers is defined in terms of addition; it is repeated addition. So, asking how many times I can add 3 to a number until I get 2014 is related to multiplication, which in turn is related to division, because division is defined in terms of multiplication.
I hope you can help!