Lambda optimization

Правка en1, от adamant, 2021-12-25 16:21:30

Hi everyone!

This time I'd like to write about what's widely known as "Aliens trick" (as it got popularized after 2016 IOI problem called Aliens). There are already some articles about it here and there, and I'd like to summarize them.


Lagrange multipliers

Let $$$f : \mathbb R^n \to \mathbb R$$$ be the objective function and $$$g : \mathbb R^n \to \mathbb R^c$$$ be the constraint function. Then the constrained optimization problem

$$$\begin{gather}f(\vec x) \to \text{extr}\\ g(\vec x)=0\end{gather}$$$

in some cases (when $$$f$$$ and $$$g$$$ are continuously differentiable) can be reduced to the unconstrained optimization problem

$$$f(x) - \lambda \cdot g(x) \to \text{extr}$$$

Here $$$\lambda \cdot g(x)$$$ denotes the dot product of a variable vector $$$\lambda$$$ and $$$g(x)$$$. The reduction means that for any solution $$$x^*$$$ of the initial problem there exists $$$\lambda^*$$$ such that $$$(x^*, \lambda^*)$$$ is the solution to the unconstrained problem. We will consider two possible interpretations of this.

Geometrically, the idea behind Lagrange multipliers is that in the extreme point $$$x^*$$$, the tangent space to the constraint surface $$$g(x)=0$$$ should lie completely within the tangent space of the contour surface $$$f(x)=f(x^*)$$$. See the illustration below for 2D case.

For linear spaces, $$$A \subset B$$$ if and only if $$$B^\bot \subset A^\bot$$$ where $$$A^\bot$$$ is an orthogonal complement space of $$$A$$$. For a tangent space, its orthogonal complement is a normal space. For the surface $$$f(x)=d$$$, normal is a line formed by the gradient vector $$$\nabla f$$$. And for the surface $$$g(x)=0$$$, its normal is a linear space formed by the vectors $$$\nabla g_1 (x), \dots, \nabla g_c(x)$$$.

Thus, for the point $$$x^*$$$ to be the solution of constrained problem, $$$\nabla f(x^*)$$$ must be a linear combination of $$$\nabla g_1(x^*), \dots, \nabla g_c(x^*)$$$. Which is exactly the extreme point condition of $$$L(x, \lambda)$$$. Thus the geometric meaning of the $$$\lambda$$$ vector is to represent the coefficients in the linear combination that forms up $$$\nabla f(x^*)$$$.

Model problem

You're given an array $$$a_1, \dots, a_n$$$. You have to pick $$$k$$$ non-intersecting contiguous sub-arrays with maximum sum of elements.

Typical solution would be to consider $$$dp(i, j)$$$ as the maximum possible answer on first $$$i$$$ elements with $$$j$$$ sub-arrays.

References

Теги lambda, aliens, tutorial, lagrange, duality

История

 
 
 
 
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en68 Английский adamant 2023-10-11 00:47:07 8
en67 Английский adamant 2022-02-13 16:55:43 33
en66 Английский adamant 2022-01-04 14:57:40 25
en65 Английский adamant 2022-01-04 05:03:24 6363 + honorable mention
en64 Английский adamant 2022-01-04 04:20:18 8 articles
en63 Английский adamant 2022-01-04 04:18:39 354 example 3
en62 Английский adamant 2022-01-04 04:00:52 748 tldr structured
en61 Английский adamant 2022-01-04 03:39:52 33
en60 Английский adamant 2022-01-04 03:38:28 721 example, part 2
en59 Английский adamant 2022-01-04 03:25:48 784
en58 Английский adamant 2022-01-04 03:18:14 1414 example
en57 Английский adamant 2022-01-04 00:21:04 4
en56 Английский adamant 2022-01-04 00:20:45 570 better code for min_conv
en55 Английский adamant 2022-01-03 15:20:41 472 clarified tldr
en54 Английский adamant 2022-01-03 03:37:09 688 code for max-conv of concave functions
en53 Английский adamant 2022-01-03 01:11:10 43 link
en52 Английский adamant 2022-01-03 01:07:50 30
en51 Английский adamant 2022-01-03 01:06:56 12
en50 Английский adamant 2022-01-03 01:02:56 1815 + example
en49 Английский adamant 2022-01-02 20:14:13 160 sections in testing convexity
en48 Английский adamant 2022-01-02 13:29:02 129
en47 Английский adamant 2022-01-02 13:26:24 104 (published)
en46 Английский adamant 2022-01-02 13:21:47 3709
en45 Английский adamant 2022-01-02 12:55:16 690
en44 Английский adamant 2022-01-02 01:54:22 24
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en30 Английский adamant 2021-12-29 05:34:22 1201
en29 Английский adamant 2021-12-29 03:47:08 332
en28 Английский adamant 2021-12-29 02:45:56 2366
en27 Английский adamant 2021-12-28 22:09:05 1383
en26 Английский adamant 2021-12-28 19:28:41 1585
en25 Английский adamant 2021-12-28 18:57:44 52
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en22 Английский adamant 2021-12-28 18:28:53 112
en21 Английский adamant 2021-12-28 18:25:09 218
en20 Английский adamant 2021-12-28 18:18:48 1224 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en19 Английский adamant 2021-12-28 17:51:33 66
en18 Английский adamant 2021-12-28 17:50:21 316 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en17 Английский adamant 2021-12-28 17:35:06 409 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en16 Английский adamant 2021-12-28 17:18:07 81 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en15 Английский adamant 2021-12-28 17:14:07 1049 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en14 Английский adamant 2021-12-28 16:12:52 96 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en13 Английский adamant 2021-12-28 16:01:48 338 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en12 Английский adamant 2021-12-28 15:54:43 903 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en11 Английский adamant 2021-12-28 04:38:22 2 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en10 Английский adamant 2021-12-28 04:35:18 118 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en9 Английский adamant 2021-12-28 04:23:41 1406 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en8 Английский adamant 2021-12-28 03:18:55 333 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en7 Английский adamant 2021-12-28 02:59:30 322 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en6 Английский adamant 2021-12-28 01:40:50 2845 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en5 Английский adamant 2021-12-27 22:08:35 26 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en4 Английский adamant 2021-12-26 19:06:11 0 Tiny change: 'blem\n\n$$f(x)→ming(x)=0f(x)→ming(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en3 Английский adamant 2021-12-26 01:42:07 1888 Tiny change: 'blem\n\n$$f(x)→extrg(x)=0f(x)→extrg(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en2 Английский adamant 2021-12-25 16:27:58 176 Tiny change: 'blem\n\n$$f(x)→extrg(x)=0f(x)→extrg(x)=0\begin{gat' -> 'blem\n\n$$\begin{gat'
en1 Английский adamant 2021-12-25 16:21:30 2909 Initial revision (saved to drafts)