# Duality gap

In optimization problems in applied mathematics, the duality gap is the difference between the primal and dual solutions. If ${\displaystyle d^{*}}$ is the optimal dual value and ${\displaystyle p^{*}}$ is the optimal primal value then the duality gap is equal to ${\displaystyle p^{*}-d^{*}}$. This value is always greater than or equal to 0 (for minimization problems). The duality gap is zero if and only if strong duality holds. Otherwise the gap is strictly positive and weak duality holds.[1]

In general given two dual pairs separated locally convex spaces ${\displaystyle \left(X,X^{*}\right)}$ and ${\displaystyle \left(Y,Y^{*}\right)}$. Then given the function ${\displaystyle f:X\to \mathbb {R} \cup \{+\infty \}}$, we can define the primal problem by

${\displaystyle \inf _{x\in X}f(x).\,}$

If there are constraint conditions, these can be built into the function ${\displaystyle f}$ by letting ${\displaystyle f=f+I_{\mathrm {constraints} }}$ where ${\displaystyle I}$ is the indicator function. Then let ${\displaystyle F:X\times Y\to \mathbb {R} \cup \{+\infty \}}$ be a perturbation function such that ${\displaystyle F(x,0)=f(x)}$. The duality gap is the difference given by

${\displaystyle \inf _{x\in X}[F(x,0)]-\sup _{y^{*}\in Y^{*}}[-F^{*}(0,y^{*})]}$

where ${\displaystyle F^{*}}$ is the convex conjugate in both variables.[2][3][4]

In computational optimization, another "duality gap" is often reported, which is the difference in value between any dual solution and the value of a feasible but suboptimal iterate for the primal problem. This alternative "duality gap" quantifies the discrepancy between the value of a current feasible but suboptimal iterate for the primal problem and the value of the dual problem; the value of the dual problem is, under regularity conditions, equal to the value of the convex relaxation of the primal problem: The convex relaxation is the problem arising replacing a non-convex feasible set with its closed convex hull and with replacing a non-convex function with its convex closure, that is the function that has the epigraph that is the closed convex hull of the original primal objective function.[5][6][7][8][9][10][11][12][13]

## References

1. Borwein, Jonathan; Zhu, Qiji (2005). Techniques of Variational Analysis. Springer. ISBN 978-1-4419-2026-3.
2. Radu Ioan Boţ; Gert Wanka; Sorin-Mihai Grad (2009). Duality in Vector Optimization. Springer. ISBN 978-3-642-02885-4.
3. Ernö Robert Csetnek (2010). Overcoming the failure of the classical generalized interior-point regularity conditions in convex optimization. Applications of the duality theory to enlargements of maximal monotone operators. Logos Verlag Berlin GmbH. ISBN 978-3-8325-2503-3.
4. Zălinescu, C. (2002). Convex analysis in general vector spaces. River Edge, NJ: World Scientific Publishing Co. Inc. pp. 106–113. ISBN 981-238-067-1. MR 1921556.
5. Ahuja, Ravindra K.; Magnanti, Thomas L.; Orlin, James B. (1993). Network Flows: Theory, Algorithms and Applications. Prentice Hall. ISBN 0-13-617549-X.
6. Bertsekas, Dimitri P. (1999). Nonlinear Programming (2nd ed.). Athena Scientific. ISBN 1-886529-00-0.
7. Bonnans, J. Frédéric; Gilbert, J. Charles; Lemaréchal, Claude; Sagastizábal, Claudia A. (2006). Numerical optimization: Theoretical and practical aspects. Universitext (Second revised ed. of translation of 1997 French ed.). Berlin: Springer-Verlag. pp. xiv+490. doi:10.1007/978-3-540-35447-5. ISBN 3-540-35445-X. MR 2265882.
8. Hiriart-Urruty, Jean-Baptiste; Lemaréchal, Claude (1993). Convex analysis and minimization algorithms, Volume I: Fundamentals. Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences]. 305. Berlin: Springer-Verlag. pp. xviii+417. ISBN 3-540-56850-6. MR 1261420.
9. Hiriart-Urruty, Jean-Baptiste; Lemaréchal, Claude (1993). "XII. Abstract Duality for Practitioners". Convex analysis and minimization algorithms, Volume II: Advanced theory and bundle methods. Grundlehren der Mathematischen Wissenschaften [Fundamental Principles of Mathematical Sciences]. 306. Berlin: Springer-Verlag. pp. xviii+346. doi:10.1007/978-3-662-06409-2_4. ISBN 3-540-56852-2. MR 1295240.
10. Lasdon, Leon S. (2002) [Reprint of the 1970 Macmillan]. Optimization theory for large systems. Mineola, New York: Dover Publications, Inc. pp. xiii+523. ISBN 978-0-486-41999-2. MR 1888251.
11. Lemaréchal, Claude (2001). "Lagrangian relaxation". In Jünger, Michael; Naddef, Denis (eds.). Computational combinatorial optimization: Papers from the Spring School held in Schloß Dagstuhl, May 15–19, 2000. Lecture Notes in Computer Science (LNCS). 2241. Berlin: Springer-Verlag. pp. 112–156. doi:10.1007/3-540-45586-8_4. ISBN 3-540-42877-1. MR 1900016.
12. Minoux, Michel (1986). Mathematical programming: Theory and algorithms. Egon Balas (forward); Steven Vajda (trans) from the (1983 Paris: Dunod) French. Chichester: A Wiley-Interscience Publication. John Wiley & Sons, Ltd. pp. xxviii+489. ISBN 0-471-90170-9. MR 0868279. (2008 Second ed., in French: Programmation mathématique : Théorie et algorithmes, Éditions Tec & Doc, Paris, 2008. xxx+711 pp. ISBN 978-2-7430-1000-3. MR2571910).
13. Shapiro, Jeremy F. (1979). Mathematical programming: Structures and algorithms. New York: Wiley-Interscience [John Wiley & Sons]. pp. xvi+388. ISBN 0-471-77886-9. MR 0544669.