>	f := (2/3)^(x-1)*(1/3);

To show that this is indeed a p.d.f., sum f  from 1 to .

>	sum(f, x = 1 .. infinity);

>	mu := sum(x*f, x = 1 .. infinity);

>	var := sum((x - mu)^2*f, x = 1 .. infinity);

>	sigma := sqrt(var);

>	sigma := evalf(sigma);

>