What is the Formulation of the backtracking?

The Formulation of the backtracking : T(X₁,X₂,...,X_i): set of all possible values for x_i+1 such that (X₁,X₂,...,X_i,X_i+1) is also a B_i+1>(X₁,X₂,...,X_i,X_i+1): bounding function If B_i+1>(X₁,X₂,...,X_i,X_i+1) is false, then the path cannot be extended to reach an answer node.