Ptolemy’s theorem

GPS satellites all orbit at the same altitude. According to the FAA, GPS satellites fly in circular orbits at an altitude of 10,900 nautical miles (20,200 km) and with a period of 12 hours. Why were these orbits chosen? You can determine your position using satellites that are not in circular orbits, but with circular […] The post GPS satellite...

A few weeks ago I wrote about the Mellin transform. Mitchell Wheat left comment saying the transform seems reminiscent of Ramanujan’s master theorem, which motivated this post. Suppose you have a function f that is nice enough to have a power series. Now focus on the coefficients an as a function of k. We’ll introduce […] The post Ramanujan’s...

Here’s a simple calculation that I’ve done often enough that I’d like to save the result for my future reference and for the benefit of anyone searching on this. A linear combination of sines and cosines a sin(x) + b cos(x) can be written as a sine with a phase shift A sin(x + φ). […] The post Linear combination of sine and cosine as phase shift...

Suppose you want to write a shell script searches the current directory for files that have a keyword in the name of the file or in its contents. Here’s a first attempt. find . -name '*.py' -type f -print0 | grep -i "$1" find . -name '*.py' -type f -print0 | xargs -0 grep -il […] The post Resolving a mysterious problem with find first appeared on...

I recently stumbled upon the Postage Stamp Problem. Given two relatively prime positive numbers a and b, show that any sufficiently large number N, there exists positive integers x and y such that ax + by = N. I initially missed the constraint that x and y must be positive, in which result is well […] The post The Postage Stamp Problem first...