A small guide on calculating the day of week fairly easily
Here Is a small guide in Disqus-formatting, that I have ‘published’ several times on the old GSMArena.com blog (with tiny variation).
UPDATE: Should in 2019 finally be more or less obsolete for wide enough windows!
A nerdy idea:
A C# class that quicly can supply a lot of primes (here limited to about the largest prime about 50G – as arrays with over 2G entries are not currently supported without splitting the arrays)
A struct that supports basic arithmetic for large accurately represented numbers in Q, by factorization into primes.
A test program that tries to find solutions for a!b!=c! with 1<a<b<c
For some reason, surprisingly few know this extremely simple formula, please share it.
I have actually seen in a carpet store an employee unrolling the carpet to measure the length!!
[The formula is simple to prove too… r=kφ, L=∫r dφ=∫kφ dφ=½k(φ2²-φ1²)=½k(φ2+φ1)×(φ2-φ1)=½(r2+r1) ×n2π=(r2+r1) × nπ , q.e.d.]