sigmoid.R
##
## s i g m o i d . R Sigmoid Function
##
sigmoid <- function(x, a = 1, b = 0) {
if (length(x) == 0) return(c())
stopifnot(is.numeric(x) || is.complex(x))
stopifnot(is.numeric(a), is.numeric(b))
a <- a[1]; b <- b[1]
1 / (1 + exp(-a*(x-b)))
}