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I'm reading this paper http://eccc.hpi-web.de/report/2016/044/download/, at page 7 you can see this equation:

$$ \sum_t \mathbb{E}_{w \in W} [|p_t - p_{t|w,w}|] = 2 \sum_t \mathbb{E}_{w \in W} [\max(0,p_t - p_{t|w,w})]$$

I think what we're using here is this: $$a+b+|a-b| = 2 \max(a,b)$$

but with $a=0$ and $b=p_t-p_{t|w,w}$ we get: $$p_t - p_{t|w,w} + |p_t - p_{t|w,w}| = 2\max(0,p_t - p_{t|w,w})$$ I know it's a bit of a silly question but I just can't figure it out, maybe I'm missing some fact about probabilities?

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  • $\begingroup$ Presumably, summing over $t$ of both $p_t$ and $p_{t|w,w}$ yields 1 so they cancel out. $\endgroup$ – Aryeh Aug 27 '16 at 18:00
  • $\begingroup$ using $b=p_t - p_{t|w,w}$ i need to substract from the right-hand side $p_{t|w,w}-p_t$, so we would have: $$\sum_t \mathbb{E}_{w \in W} (p_{t|w,w}-p_t)$$, i can split this in two sums, the first giving $$\sum_t \mathbb{E}_{w \in W} p_{t|w,w} = \sum_t p_t$$ and the other give the same so they cancel out, is this correct? $\endgroup$ – Marco Aug 27 '16 at 19:02
  • $\begingroup$ I didn't read the paper but if $t$ is indeed the "index" of the distribution (i.e., summing over $t$ yields 1) then that's correct. $\endgroup$ – Aryeh Aug 28 '16 at 18:13
  • $\begingroup$ I suggest closing the question. It's not research level and I believe the answer is resolved in the comments. $\endgroup$ – Aryeh Aug 29 '16 at 20:07
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This uses the fact that the L1-norm between two probability distributions is equal to twice their total variation distance.

See https://en.wikipedia.org/wiki/Total_variation_distance_of_probability_measures

To prove the equality you take the subset A in the definition to be the set of points where $$p_t > p_{t|w,w}.$$

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