It isn't the page you're looking for, but it is an interactive demonstration of a Dirichtlet kernel
Looking for the old 404 page of a Fourier series?
var c = document.createElement('canvas'); var w = window.innerWidth / 4, h = window.innerHeight / 2; c.width = w; c.height = h; var ctx = c.getContext('2d'); ctx.fillStyle = '#fff'; ctx.strokeStyle = '#005791'; ctx.lineWidth = 2; function wave(t, n, f) { var val = 0; for (var k = 1; k < n; k ++) { val += Math.sin((k + (1/2)) * t) / Math.sin(t / 2); } return val; } function draw(n, f, a) { ctx.fillRect(0,0,w,h); ctx.beginPath(); for (var t = -Math.PI; t < Math.PI; t += 0.005) { var y = wave(t, n, f) ctx.lineTo((Math.PI + t) * (w / Math.PI / 2), (h/1.1) - (y * 1)); } ctx.stroke(); } function drawAt(n) { draw(n, 0.2); document.body.style.backgroundImage = 'url(' + c.toDataURL(); ')'; document.body.style.backgroundSize = '' + (w / 2) + 'px ' + (h / 2) + 'px'; } document.onmousemove = function(e) { drawAt((Math.floor(e.pageX * 20 / window.innerWidth)) + 1); } drawAt(0.5);