e^pi vs pi^e

Hi Prof Lee,

Thanks for the lunch (including the advice part). I came up with some ideas about this brain teaser —

Q: which is bigger e^pi vs pi^e

One solution I can think of is, suppose e has a value close to 2 and pi is much larger.

Suppose e = 2 and pi = 10. Clearly e^pi wins.

Another way is, define 2 functions

f1(x) = 2.718281828^x and find the growth rate when x is slightly above e. This growth rate is e^x,

f2(x) = x^2.718281828 and find the growth rate when x is slightly above e. This grow rate is e/x * x^e, which is smaller, since x is slightly bigger than e.

Therefore, f1 grows faster than f2, over the range of (e , 3.15). Therefore e^pi wins.

pick line passing the most points – solved

Q: Given N points with positive integer coordinates, find the straight line passing through the most points
A: For each of (N*N-N)/2 pairs of points, compute a Line object identified by 2 numbers:
* a slope S = (y2 – y1)/(x2 – x1)
* intercept on y-axis.

So these 2 numbers can be computed easily from any pair.

Save each Line object as key in a hashmap. When a pair gives a Line that’s already seen, increment its count.

Intercept formula y_inter(int, int, int, int) can be assumed to exit. Writing this function isn’t relevant to a coding interview:

Suppose this value is y3, so the incept point is (0,y3), so

(y3-y1)/(0-x1) = S, so y3 = y1 – S x1

2 overlapping rectangles

bloomberg-lp-interview-question.

Q: You have a rectangle a and b. Determine if the two rectangles overlap. That is at least some part of either rectangle should be within the other.

I will assume the coordinates are given.

%%A: find both centers.
– case 1: identify box A’s corner that’s closes to B’s center. Check if this corner is inside B.

– case 2: if they lie on one vertical line, then only the bottom border (of upper box) vs top border (of lower box) matters.

PDE/SDE

I have seen a few PDE/SDE combos. There’s a pattern among them.

The SDE tends to describe a process as a signal-noise description. It is actually rather precise, as precise as it gets — you can compute the exact probability of the “particle” falling into a range at a given time t.

However, the SDE won’t enable us to compute today’s price as an equation can, such as “sin(x) + log(x) – sqrt(x) = pi”. Reason? The dW term is an obstacle. Therefore, we need to somehow get rid of the dW term. We end up with a differential equation, often a PDE. If there are sufficient boundary conditions, we could solve the equation to get a precise time-0 price

PDE solver – phrasebook

[[Numerical methods and optimization in finance]] has simple examples

of a PDE numerical solver. This book is more in-depth than [[basic

black-scholes]]

I think the Daniel Duffy book also covers Finite Difference method

Brute-force — often there's no other solution (like mathematician

tackling a proof) to a PDE. Numerical solution is kinda versatile.