A certain segment of the sky contains Z starts. Each star is either a red giant with probability 1/3 or a white dwarf with probability 2/3, independently from the other stars.
(a) If we condition on the event {Z=4}, what is the joint p.m.f of the red giants X and the white dwarves Y? Fill in a 5*5 table with the
(f(x,y))from x,y=0 to 4 values
(b) Show that if Z~POI(6), then the random variables X and Y are indipendent and have Poisson distribution by calculating the joint p.m.f of X and Y