Nothing but Primes

Numbers sink through deep water. As they sink, they come apart into their prime factors. 12 becomes 2 and 2 and 3. The primes drift to a number line at the edge of the screen, light up, and pass on. Some numbers do not come apart. Divisible only by 1 and themselves — primes. Those stop where they are and glow amber. - You do nothing No arithmetic. Tapping does not make anything go faster. There is no winning and no ending. Touch the screen once to begin, and then watch. - The number line is walked, not sampled Numbers are not thrown out at random. They are taken in order, one at a time, from 2. So twin primes — two apart — and prime deserts, long stretches with none at all, appear exactly as they really are. Nothing here is staged. - Amber means prime That colour is used nowhere else in the app. You do not have to learn which numbers are prime. The glowing ones are. - A codex When a number with some property drifts past, a card is added. Taxicab 1729, perfect numbers, amicable numbers, Mersenne primes, prime deserts, the sieve of Eratosthenes, semiprimes and RSA — twenty in all. Every prime you have passed, and the whole number line you have walked, stay readable. - You can slow it down You can make it faster, and you can always put the speed back to how it began. Nothing you have built up is lost by doing so. - It stops, eventually When the numbers grow too large, the app stops. Either it reaches the edge of what a 64-bit integer can hold, or it meets a number it cannot factor within the work allowed. Calling something prime when it is not known to be prime is not an option, so it stops and tells you why. That place is the foot of the wall the internet's encryption stands on: instant to multiply, forbidding to take apart. Everything is computed on your device. The network is used only to show ads.