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