#model including both parameters and their interaction with "*"
m1 <- lm(Sepal.Length ~ Petal.Width * Petal.Length, data = iris)
coef(m1)
(Intercept) Petal.Width Petal.Length Petal.Width:Petal.Length
4.5771709 -1.2393154 0.4416762 0.1885887
#model including both pars and interaction (all terms spelled out)
m2 <- lm(Sepal.Length ~ Petal.Width + Petal.Length + Petal.Width:Petal.Length, data = iris)
coef(m2)
(Intercept) Petal.Width Petal.Length Petal.Width:Petal.Length
4.5771709 -1.2393154 0.4416762 0.1885887
#model only including the interaction
m3 <- lm(Sepal.Length ~ Petal.Width:Petal.Length, data = iris)
coef(m3)
(Intercept) Petal.Width:Petal.Length
4.9704818 0.1506457