dgamma(a,b): Gamma distribution with parameter a and b.
f(x)= b^a/( Gamma(a)) x^(a-1) e^-(b*x)
mean=a/b
variance=a/b^2
== R ==
dgamma(x,shape=a,scale=s): Gamma distribution with parameter a and s.
f(x)= 1/(s^a Gamma(a)) x^(a-1) e^-(x/s)
mean=a*s
variance=a*s^2