nbpr(n):=(s:[], for k from 2 thru n do (if primep(k) then s:append(s,[k])),s);
nbpr(20);
nbnbpr(n):=(j:0, for i from 2 thru n do (if primep(i) then j:j+1),j);
nbnbpr(100);
plot2d(['(float(nbnbpr(entier(x)))),x/log(x),'(romberg(1/log(t),t,2,x))],[x,20,500],[y,0,500]);