# Why do probablity distribution with orthogonal suppor have maximal Kolmogorov distance?

Can anyone explain why the $$l_1$$ distance has the property that probability distributions $$P,Q$$ with orthogonal support (meaning that the product $$p_iq_i$$ vanishes for each value of $$i$$) are at a maximal distance from each other? Consider $$\|P-Q\|_1=\frac{1}{2}\sum_{i=1}^{N}|p_i-q_i|$$

Note that if $$p_{i}q_{i} = 0\,\,\forall i$$, then for all $$i$$ either $$p_{i} = 0$$, $$q_{i} = 0$$, or both are $$0$$.

Divide $$\{i\} = \{1,\ldots,N\}$$ into those $$i$$ for which these three different things happen: $$\{N_{p_{i}}\} = \{i|p_{i} = 0\}$$, $$\{N_{q_{i}}\} = \{i|q_{i} = 0\}$$, $$\{N_{pq_{i}}\} = \{i|p_{i} = q_{i} = 0\}$$.

Then $$\begin{equation} \begin{split} \|P-Q\|_{1} =& \frac{1}{2}\sum_{i=1}^{N}|p_{i}-q_{i}| \\ =& \frac{1}{2}\sum_{i\in N_{p_{i}}}|p_{i}-q_{i}| + \frac{1}{2}\sum_{i\in N_{q_{i}}}|p_{i}-q_{i}| + \frac{1}{2}\sum_{i\in N_{pq_{i}}}|p_{i}-q_{i}| \\ =& \frac{1}{2}\sum_{i\in N_{p_{i}}}|q_{i}| + \frac{1}{2}\sum_{i\in N_{q_{i}}}|p_{i}| + \frac{1}{2}\sum_{i\in N_{pq_{i}}}|0| \\ =& \frac{1}{2}\Big(\sum_{i \in \{N\}}q_{i} + \sum_{i \in \{N\}} p_{i}\Big) = 1, \end{split} \end{equation}$$

where the last line follows from the fact that $$1 = \sum_{i\in \{N\}}|q_{i}| = \sum_{i\in N_{p_{i}}}|q_{i}| + \sum_{i\in N_{q_{i}}}|q_{i}| + \sum_{i\in N_{pq_{i}}}|q_{i}| = \sum_{i\in N_{p_{i}}}|q_{i}| + 0 + 0 = \sum_{i\in N_{p_{i}}}|q_{i}|,$$ and likewise for $$\sum_{i}p_{i}$$. That is, we can add these other terms of $$q_{i}$$ ($$p_{i}$$) because they are $$0$$ by construction anyway.

This is the maximum value that $$\|P-Q\|_{1}$$ can take for any $$P$$ and $$Q$$, because $$|p_{i} - q_{i}| \leq |p_{i}| + |-q_{i}| = p_{i} + q_{i}$$ for any $$p_{i},q_{i}$$ imaginable, and as such:

$$\|P-Q\|_{1} = \frac{1}{2}\sum_{i=1}^{N}|p_{i}-q_{i}| \leq \frac{1}{2}\big(\sum_{i=1}^{N}p_{i}+q_{i}\big) = \frac{1}{2}\sum_{i=1}^{N}p_{i} + \frac{1}{2}\sum_{i=1}^{N}q_{i} = \frac{1}{2}+\frac{1}{2} = 1$$

• can you please explain why the maximum value of $\|P-Q\|_1$ is 1? – Sakh10 Jan 28 at 10:56
• Can I use minkowski inequality to show the maximum value equals to 1? – Sakh10 Jan 28 at 12:19
• You can use the triangle inequality on each of the terms in the sum to show that it is bounded above by $1$. – Rammus Jan 28 at 12:42
• @Rammus thanks! I was just adding it to my answer too:) – JSdJ Jan 28 at 12:43
• @Sakh10 Please see the added text – JSdJ Jan 28 at 12:43