请问谁知道plm包里单位根检验的例子中，最后一句里"inv ~ 1"是什么含义吗，短波浪线不是写回归方程时用的吗，这里怎么理解呀

data("Grunfeld", package = "plm")
y <- data.frame(split(Grunfeld$inv, Grunfeld$firm)) # individuals in columns
purtest(y, pmax = 4, exo = "intercept", test = "madwu")
## same via formula interface
purtest(inv ~ 1, data = Grunfeld, index = c("firm", "year"), pmax = 4, test = "ips")