Closure Operation

1.              Substitution.

        Suppose functions g(y₁, ..., y_m), h₁(x₁, ..., x_n),..., h_m(x₁, .... x_n) are given.

The function f defined by f(x₁, .... x_n) = g(h₁(x₁, ..., x_n),..., h_m(x₁, .... x_n)) is said to be obtained from g and h₁, .... h_m by substitution.