diff --git a/assignments/week1/1.pdf b/assignments/week1/1.pdf index a65baf2..b5e39f4 100644 Binary files a/assignments/week1/1.pdf and b/assignments/week1/1.pdf differ diff --git a/assignments/week1/1.tex b/assignments/week1/1.tex index 715613b..699d7fa 100644 --- a/assignments/week1/1.tex +++ b/assignments/week1/1.tex @@ -133,5 +133,32 @@ Each set is definitely finite, because they all contain just one element. Finally, the union of the collection of sets is equal to $\N$, which is not a finite set. \exercise* +\begin{tcolorbox} + \begin{enumerate}[label=\emph{\alph*)}, wide] + \item Compute $f(4/15)$. Find $q$ such that $f(q) = 108$. + \item Use the \textbf{Theorem} to prove that $f$ is a bijection. + \end{enumerate} +\end{tcolorbox} + +See the assignment PDF for the full assignment specification and theorem. + +\begin{enumerate}[label=\emph{\alph*)}, wide] + \item $\frac{4}{15}$, if written as a product of prime factors, is equal to $\frac{2^2}{3^1*5^1}$. + Since this fraction is not a natural number, we have to use the second part of the definition of $f$. + So, $f(q) = 2^{2*2} * 3^{2 * 1 - 1} * 5^{2 * 1 - 1} = 240$. + + For the inverse of $f$, it is still necessary to compute the factorization in prime numbers. + Using the powers of the primes we can deduce whether the prime present is, if applicable, + part of either the numerator or the denominator. + $180 = 2^2 * 3^2 * 5^1$. Because of the way $f$ is defined, we know that all the prime factors with an even + power are part of the numerator and all prime factors with an odd power are part of the denominator (except 1, + which just maps to itself). When we backtrack using this information, we then get the following fraction: + $\frac{2^1 * 3^1}{5^1} = \frac{6}{5}$. + \item In order to prove that $f$ is a bijection, we have to prove that $f$ is injective and surjective. + \begin{description} + \item[Surjectivity:] + \item[Injectivity:] + \end{description} +\end{enumerate} \end{document}