kill(all)$

tau:1/sigma^2;

diff(tau,sigma);

assume(sigma>0);

f(x,mu,sigma):=1/sqrt(2*pi*sigma^2)*%e^(-(x-mu)^2/(2*sigma^2));

f(x,0,sigma);

diff(f(x,0,sigma),x,1);

diff(f(x,0,sigma),x,2);