Instead of expanding the slow part of f ( Γ Γ ) in the continuous set Φ Φ a ( Γ Γ ) = δ δ ( A ( Γ Γ ) − − a ) of functions one also might use some enumerable set of functions Φ Φ n ( A ( Γ Γ ) ) . If these functions constitute a complete orthonormal function set then the projection operator simply reads