Hi, Rusers:

Thanks for answering my last questions. I am frustrated in plotting a trinomial pmf function 

f(x,y | n, pa, pb) = factorial(n)/ (factorial(x) * factorial(y) * factorial (n-x-y))* pa^x * pb^y * ((1-pa-pb)^(n-x-y))

obviously it is a bivariate function of x and y. But I have put a lot of time on this.

**********************************
x <- seq(0, n, len = n/2+1) # for now I set it to n/2 to control x+y <= n
y <- seq(0, n, len = n/2+1)
f = factorial(n)/ (factorial(x) * factorial(y) * factorial (n-x-y))* pa^x * pb^y * ((1-pa-pb)^(n-x-y))
wireframe(f ~ x * y, shade = TRUE)
**********************************

well, but it plots nothing out.

I wonder if you could help me? Seems R is hard to learn without your help.

Many thanks,

Sun
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