diff --git a/latex/content.tex b/latex/content.tex index 433621d..2003c8f 100644 --- a/latex/content.tex +++ b/latex/content.tex @@ -145,17 +145,19 @@ Let $ A_k = \{ U_1 + U_2 + \ldots + U_{k-1} < 1 \}$. Hence, & = P(A_{k-1}) - P(A_k) \qquad \text{ (as $ A_k \subset A_{k-1} $)} \\ \end{align*} -We will try to show that $ \forall k \geq 2 $, $ P(A_k) = \frac{1}{k!} $. -To do so we will use induction to demonstrate that +We will try to show that $ \forall k \geq 2 $, $ P(A_k) = \frac{1}{k!} $. To do +so, we will use induction to prove the following proposition \eqref{eq:induction}, +$ \forall k \geq 2 $: \begin{equation} \label{eq:induction} - P(U_1 + U_2 + \ldots + U_{k-1} < a) = \frac{a^k}{k!} \qquad \forall a \in [0, 1], \forall k \geq 2 + \tag{$ \mathcal{H}_k $} + P(U_1 + U_2 + \ldots + U_{k-1} < a) = \frac{a^k}{k!} \qquad \forall a \in [0, 1], \end{equation} Let us denote $ S_k = U_1 + U_2 + \ldots + U_{k-1} \qquad \forall k \geq 2 $. -\paragraph{Base cases} $ k = 2 $ : $ P(U_1 < a) = a $ +\paragraph{Base cases} $ k = 2 $ : $ P(U_1 < a) = a \neq \frac{a^2}{2}$ supposedly proving $ (\mathcal{H}_2) $. $ k = 2 $ : \[ P(U_1 + U_2 < a) = \iint_{\cal{D}} f_{U_1, U_2}(x, y) \cdot (x + y) dxdy \] @@ -184,6 +186,43 @@ Hence, \end{align*} +\paragraph{Induction step} For a fixed $ k > 2 $, we assume that $ + (\mathcal{H}_{k-1}) $ is true. We will try to prove $ (\mathcal{H}_{k}) $. + +\[ + P(S_{k-1} + U_{k-1} < a) + = \iint_{\cal{D}} f_{S_{k-1}, U_{k-1}}(x, y) \cdot (x + y) dxdy \\ +\] +where $ \mathcal{D} = \{ (x, y) \in [0, 1]^2 \mid x + y < a \} $. +As $ S_{k-1} $ and $ U_{k-1} $ are independent, +\[ + P(S_{k-1} + U_{k-1} < a) + = \iint_{\cal{D}} f_{S_{k-1}}(x) \cdot f_{U_{k-1}}(y) \cdot (x + y) dxdy \qquad \\ +\] + +$ (\mathcal{H}_{k-1}) $ gives us that $ \forall x \in [0, 1] $, +$ F_{S_{k-1}}(x) = P(S_{k-1} < x) = \frac{x^{k-1}}{(k-1)!} $. + +By differentiating, we get that $ \forall x \in [0, 1] $, + +\[ + f_{S_{k-1}}(x) = F'_{S_{k-1}}(x) = \frac{x^{k-2}}{(k-2)!} +\] + +Furthermore, $ U_{k-1} $ is uniformly distributed on $ [0, 1] $, so +$ f_{U_{k-1}}(y) = 1 $. + +\begin{align*} + \text{Hence, } + P(S_{k-1} + U_{k-1} < a) + & = + & = \frac{a^{k}}{k!} +\end{align*} + + + + + \section{Complexity and implementation optimization} The NFBP algorithm has a linear complexity $ O(n) $, as we only need to iterate