A wave of every integer period ripples out from the origin, crossing the axis at its multiples. Point at an integer to darken the waves passing through it — its divisors. Primes are the lonely numbers touched by only two waves. Click to freeze a picture and paint another — up to three colours; a fourth click clears. Scroll to zoom. The 1 key stops the ever-present unit wave from highlighting; d cycles what lights up: divisors (waves crossing the line at your number on the axis), every wave gently, waves crossing above the axis, or below. The last three colour by quadratic residues mod your number — blue squares, orange non-squares, violet sharing a factor — and there a click locks the number (click again to release).