NOTE: The professor in the slides defined the result of a Relaxation as a LB (Lower Bound), this is true as long as the original problem is a minimization problem, the feasible solution space of a relaxation is bigger than that of the original problem, so there is more solution to choose from and in the minimization problem we can find a better solution which will be LOWER than the optimal solution of the original problem.
NO - Upper & Lower Bound
Link to original
NO - Relaxing & Restricting
References:
Link to original
NO - Duality
NO - Weak Duality
Link to original
NO - Strong Duality
Link to original
Link to original
NO - Weak Duality
Link to original
NO - Strong Duality
Link to original