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3blue1brown
3blue1brown

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Adding random variable, and continuous convolutions (Early view for patrons)

Hey everyone,

Continuing with the theme of probability videos, which I plan to eventually coalesce into a proper series sometime this year, this next video covers adding random variables. That is to say, convolutions.

Initially, I intended for this to be "part 2" to the convolution video from a few months ago, this time focused more exclusively on the continuous case. It builds up two visualizations, and for each one, it's very helpful to have freshly seen what it looks like for a simple discrete example (rolling a pair of dice is irresistibly relatable). The ultimate plan is for this to become a chapter in a probability series, and it would be a bit awkward to make that other video a prerequisite or to insert it into the series, given that it was mostly not about probability. So at the risk of a bit of redundancy, I decided to include the discrete case in this video, which is part of why this video took a few more weeks than expected.

I had also planned on ending the video by walking through how the diagonal slice point of view lends itself to a clean visual intuition for why convolving two Gaussian distributions gives another Gaussian, and how this relates to the central limit theorem. After incorporating the warmup examples, it was all running a bit too long, so that will be its own video. The aim is to get that out to you pretty soon.

Separating out that example may actually be the best way to do it anyway, though, because hopefully many people will be able to predict what that argument looks like, and rediscovery is always much more fun than being told something directly. A gap between videos does more to encourage someone to try it out themselves than a simple "pause and ponder".

As always, let me know if you spot any errors!

Grant

Adding random variable, and continuous convolutions (Early view for patrons)

Comments

At 25:26, why not point to Pythagoras to explain that the step is sqrt(2) times the step along the axis?

Ronald Landheer-Cieslak

Excited for the grand reveal!

Aman Karunakaran

I can try to rewrite the on-screen note at 25:15 to be more helpful. Is there a part of it you would want changed, or find most confusing?

3blue1brown

This was a great video - I was able to anticipate where you were going with the diagonal slices just before you did so and had a satisfying "aha!" when you revealed it. I would still like to understand the sqrt(2) aspect of the area under the diagonal slices - it makes sense geometrically, but I don't fully understand it.

Peter Freese

There's a weird audio artifact at 24:44.

23:37 you say 2 down to -2, but then proceed to only go down to -1.5

C.J. Smith

Great video. Waiting eagerly for the next part! Perhaps you can also contrast repeated convolutions in the continuous case with rolling more than two dies. (You do contrast a single convolution in the continuous case with rolling two dies.)

Akash Kumar

16:10

Benjamin Bailey

Very nice video, and stunning visualizations as usual! I fear though that people less used to *pdf* would not internalise that the result of the convolution is a *pdf* too as it is the result of an integral... So maybe a mention/example of use to calculate the probability of X + Y \in [a,b] might be useful...

Minor typo:a t minute 26:16 the late term in the exponential should be '\sigma_2^2' and not '\sigma_1^2'


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