###Assuming beta/alpha = 400###

library(invgamma)
values.alfa <- seq.(1,500, by = 1)

a <- pinvgamma(600, shape = values.alfa, scale = 400*values.alfa)

> a <- pinvgamma(600, shape = values.alfa, scale = (400*(values.alfa)))
> a
  [1] 0.9999958 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000
  [8] 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000
 [15] 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000
 [22] 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000 1.0000000