File:Euclid's Orchard (large).svg

int gcd(int a, int b) {
    int t;
    while(b > 0) {
        t = a;
        a = b;
        b = t % b;
    }

    return a < 0 ? -a : a;
}

size(10cm,10cm);

int ndots = 11;
draw((0,0)--(ndots - 0.5,0),Arrow);
draw((0,0)--(0,ndots - 0.5),Arrow);

for(int i = 1; i < ndots; ++i) {
    for(int j = 1; j < ndots; ++j) {
        pair tree = (i, j);
        path circ = circle(tree, 0.35);

        if(gcd(i, j) == 1) {
            fill(circ, heavyblue);
        } else {
            fill(circ, heavyred+opacity(0.2));
        }
    }
}