![]() ![]() Just like how Apple became a full fledged profitable company, the simple factorial, !, became the atom of an entire field of mathematics: combinatorics.įorget everything, let's start thinking from the bottom up. Factorials, Permutations, and Combinations were born out of mathematicians playing together, much like how Steve Jobs and Steve Wozniak founded Apple playing together in their garage. ![]() This time around, we're building intuition for permutations and combinations.įor example, do you know why the formula for a combination is (n C r)? Where did this come from? And why are factorials used here? So, from time to time, I indulge myself in an exercise of deriving things from the source, and building intuition for how things work. My mental framework isn't complete, so I decide to just remember it.Īs you can imagine, this isn't ideal. Not doing this is usually the source of confusion: if I don't understand how things work, I don't know where to hang the concepts. To understand a problem, get to the core of it, and reason up from there. I'm a big fan of first principles thinking. The way to order r items out of n is to first choose r items out of n, and then order the r items ( r! )Īnd, this means (n P r) = n! / (n-r)! and (n C r) = n! / ( (n-r)! * r! )īut do you want to know how to remember this forever? This gives rise to the familiar identity: (n P r) = (n C r) * r! If you have too many bells, you'd first choose them, and then think about ordering them. You're figuring out the best order to ring them in.Ī combination is the choice of bells. Let's take ringing bells in a church as an example.Ī permutation is an ordering of the bells. ![]()
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