# one-line

## Factorial

* the number of ways to order n unique items.
* n! = (n)(n-1)...(1)
* 0!=1, since there is only one way to order 0 items.

## Multiply

* rescale
* multiplication

## $$2 ^ x$$

* step growth = next step, double the current

## $$e ^ x$$

* natural growth = consider each part, grow little by little

## Statistics

variance and standard deviation = dispersion

expected value = long-term average

covariance = two variables have same changes or not

## Binomial / Bernoulli / Poisson

皆是Yes or No events

* binomial = discrete occurrences among discrete trials. (擲骰子10次, $$n = 10$$)
* poisson =  discrete occurrences among a continuous domain. (5年一次地震, $$n \rightarrow \infty$$)
* bernoulli = single Binomial event