set.seed(1)
x = runif(100, 0, 5)
y = ifelse(x<1, 0, ifelse(x<2, 1, ifelse(x<3,0, ifelse(x<4,1,0))))
plot(x,y, pch=20)

LM = lm(y~x)
summary(LM)          ## Output simplified

Residuals:
    Min      1Q  Median      3Q     Max
-0.5214 -0.5106  0.4829  0.4921  0.4963

Residual standard error: 0.5049 on 98 degrees of freedom
Multiple R-squared:  0.0001441, Adjusted R-squared:  -0.01006
F-statistic: 0.01413 on 1 and 98 DF,  p-value: 0.9056

LM2 = lm(y~poly(x,2))
summary(LM2)      ## Output simplified

Residuals:
    Min      1Q  Median      3Q     Max
-0.7255 -0.3478  0.3062  0.4062  0.5495

Residual standard error: 0.4659 on 97 degrees of freedom
Multiple R-squared:  0.1576,    Adjusted R-squared:  0.1402
F-statistic:  9.07 on 2 and 97 DF,  p-value: 0.0002448

library(rpart)
RP = rpart(y ~ x)
summary(predict(RP) - y)
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max.
      0       0       0       0       0       0