The Y combinator is the following term of the untyped λ\lambdaλ-calculus.
Y≜λf.(λx.f(xx))(λx.f(xx))\mathbf{Y} \triangleq \lambda f. (\lambda x. f(xx)) (\lambda x. f(xx)) Y≜λf.(λx.f(xx))(λx.f(xx))
Please expand.