Number Lab

Small machines for famous number facts. Each one does the arithmetic live in your browser, so you can poke at it, break it, and watch it work.

Kaprekar's constant

Sort, subtract, repeat. Unless its digits are all the same, every four-digit number lands on 6174 within seven rounds. Watch all 10,000 run at once.
Reverse and add

Write a number backwards, add, repeat until it reads the same both ways. 89 takes 24 steps. 196 has been taken to a billion digits without getting there. Watch your browser try.
Goldbach's conjecture

Every even number above 2 seems to be a sum of two primes. Fold the number line to find every way, then count them for all 49,999 even numbers from 4 to 100,000.