I am often amazed that one can run Maxima on an Android phone (Maxima on Android), including Gnuplot graphics and MathJax math formulae:
plot3d ([cos(x)*(3 + y*cos(x/2)), sin(x)*(3 + y*cos(x/2)), y*sin(x/2)], [x, -%pi, %pi], [y, -1, 1], ['grid, 50, 15]);
/* gradient of the log-likelihood for a time-dependent accelerated failure time model */
assume(t>0)$ H:-log(S(integrate(exp(alpha+beta*x(u)),u,0,t))); diff(delta*diff(H,t)-H,beta);