## Dynamic Models in Biology, Stephen Ellner and John Guckenheimer
## Illustrates R-files defining R functions with several inputs
##  lambda is a positive scalar, la and fa are vectors of the same length
## f is the Euler-Lotka sum 

eulot <- function(lambda, la, fa) {
  age <- 0:(length(la)-1)
  y <- lambda^-(age+1)
  f <- sum(y*la*fa) - 1
  f
}