1 Introduction
Scientists and engineers rely on accurate mathematical models to quantify the objects of their studies, which are often highdimensional. Unfortunately, highdimensional models are inherently difficult, i.e. when observations are sparse or expensive to determine. One way to address this problem is to approximate the original model with fewer input dimensions. Our project goal was to recover a function that takes inputs and returns one output, where is potentially large. For any given tuple, we assume that we can observe a sample of the gradient and output of the function but it is computationally expensive to do so. This project was inspired by an approach known as Active Subspaces, which works by linearly projecting to a linear subspace where the function changes most on average. Our research gives mathematical developments informing a novel algorithm for this problem. Our approach, Active Manifolds, increases accuracy by seeking nonlinear analogues that approximate the function. The benefits of our approach are eliminated unprincipled parameter, choices, guaranteed accessible visualization, and improved estimation accuracy.
2 Related Work
Dimension reduction, broadly defined, is the mapping of potentially high dimensional data to a lower dimensional space. Dimension reduction techniques can be categorized into two main categories, projective methods and manifold modeling
[1]. Dimension reduction techniques are widely used across many domains to analyze highdimensional models or highdimensional data sets because they allow important lowdimensional features to be extracted and allow for data visualization. The most commonly known and used projective method is Principal Component Analysis (see
[2]). The method that inspired our work, Active Subspaces, can also be considered a projective method. The Nyström method (see [3]) and related variations rely on eigenvalue problems and compromise the bulk of manifold modeling techniques. Our method, Active Manifolds, departs from the use of projective and spectral methods but is a manifold modeling method.
2.1 Active Subspaces
We chose to study Active Subspaces because it is a dimension reduction technique that reduces the dimension of the input space while respecting the output, its applicability to a wide range of functions (), and because of its accessibility to scientists and engineers with a limited mathematical background. The Active Subspaces method finds lowerdimensional subspaces of the domain by finding the directions in which the function changes the most on average. The Active Subspaces method has two main limitations. First, many functions do not admit a linear active subspace, e.g. . Second, the linearity of active subspaces and projections is restrictive and can increase estimate error.
Below is a brief description of the Active Subspaces algorithm.

Sample at random points

Find the directions in which changes the most on average, the active subspace. This is done by computing the eigenvalue decomposition of the matrix

Perform regression to estimate along the active subspace to obtain (this requires sampling at random points ).

Given a new point , project to the active subspace and use to obtain the value .
3 Results
3.1 Theory
Recall that arc length of a curve is given by
Let , where . Assume is . We seek
over all functions , and (constant speed).
Notice the integrand can be expressed as
where is the angle between and . Trivially, this quantity is maximal when , indicating that and are collinear and point in the same direction. Thus,
(1) 
Definition 3.1.
Let and and . We say that is an active manifold of if and only if, for all charts on , condition (1) is satisfied when .
Lemma 3.2.
Given and an initial value , there exists a unique solution to the system of firstorder differential equations described in (1).
Proof.
For the following theorem, let

= range

be a fixed critical point of

be the deleted attracting basin of



[row sep=huge, column sep = huge]
& X [rd, ”f”] [d, ”π”] &
R [ur, ”x(t)”] [r, ”π∘x”] & X/∼[r, ”~f”] & R
Commutative diagram for Theorem 3.3
Theorem 3.3.
Proof.

[label=()]

Realize as the single chart for induced by , thus is a 1dimensional submanifold of .

Realize given by is continuous so is a manifold with a single chart .

Realize is a bijection. It follows that is a diffeomorphism from to since charts on and are induced by .
∎
Further, the Implicit Function Theorem implies that is a dimensional manifold and orthogonally intersects . We refer to such a manifold as an Active Manifold (denoted ). Thus, we can realize an by a numerical solution to (1).
3.2 Active Manifold Algorithm Description
The Active Manifolds algorithm has three main procedures:

Building the Active Manifold

Approximating the function of interest,

Projecting a point of interest to the Active Manifold
3.2.1 Building the Active Manifold
For a given function, where , we describe below a process to build a corresponding active manifold. The active manifold will be a onedimensional curve in the hypercube that moves from a local minimum to a local maximum. We define a grid with spacing size then compute at each grid point. To build the active manifold, we use a modified gradient ascent/descent scheme with a nearest neighbor search.

Construct an dimensional grid with spacing size .

[label*=0.]

Compute at each grid point.

Normalize samples.


Given an initial starting point , use a gradient ascent/descent scheme with a nearest neighbor search to find a numerical solution to
with the samples from step 1. The set is an active manifold on .

[label*=0.]

While the active manifold builds, save the number of steps and functional values corresponding to the closest grid point at each step as ordered lists and .

Scale by so that

3.2.2 OneDimensional Function Approximation
To obtain a onedimensional approximation , perform regression on the points of . A major benefit of our method is that the set can be easily plotted and serves as a visual aid to help choose a best fit model.
3.2.3 Traversing the Level Set
Given a point , we would like to find , where and . This requires an iterative process that uses the orthogonal directions of to travel along the level set corresponding to , until we intersect the active manifold. For the following algorithm, we assume that has been normalized and tolerance and step size have been selected.

Given a point , find for (closest point on the manifold to )

Find .

While , let .

Parameterize the line segment, , between and , where is the next closest point on the manifold to .

Determine such that is orthogonal to .

Determine such that .

Evaluate .
3.2.4 Algorithm Notes

One may be enticed to minimize a distance function , for example
by letting
but this is computationally inefficient because now we must compute along with . Instead, we recommend computing and searching for the minimum or using some other nearest neighbor search.

If , it is convenient to express , and is an orthonormal basis for , it is helpful to express

It is convenient to let

The point on for which is orthogonal to can be determined by solving for t in
. Solving for gives
3.3 Empirical Results
For proof of concept and comparison to methods in [5], we proceed with data synthesized from two functions,
For each example, we are interested in how well two functions, one fit to the the Active Subspace (AS) and one fit to the Active Manifold (AM), recover the values of the function for points outside of the AS and AM. We build the AS and AM and calculate the average error for a set of random test points. The following experimental set up was observed for each example.

Define a uniform grid on with 0.05 point spacing.

Evaluate the gradient of each function, computed analytically, at each grid point.

Build the AS and AM using the gradient.

Fit the AS and AM with a polynomial ( quartic, quintic).

Draw 100 random samples and map them to the AS and AM.

Evaluate at each sample point and at the corresponding projection point.
Upon completing the experiments, average absolute errors between and were computed.
Manifold Error  

Subspace Error 
Notice that the AM reduces average absolute error, by an order of magnitude, in both examples.
In [6] the authors investigate an active subspace in a 5dimensional singlediode solar cell model. We have followed by reproducing their results with their data, while also implementing the Active Manifold algorithm. Again, we are interested in comparing estimation errors. The experiment included 10,000 randomly sampled points from . This sample was partitioned into two randomly ordered sets containing 8,000 and 2,000 points, used for training and testing, respectively. After an eight fold Monte Carlo simulation, the mean of the average absolute error was computed.
Manifold Error  

Subspace Error 
Again, the AM reduces average absolute error, by an order of magnitude
4 Conclusions and Future Work
The primary issue to be addressed in future work is determining the most appropriate way to choose an active manifold. What happens when a level set never intersects the active manifold? Should a new manifold be chosen? For future work, we propose a parallel program implementation that may run the algorithm for two or more manifolds. Future work will also seek error bounds and computational complexity estimates for the algorithm.
Acknowledgements

This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the program SULI.

ORNL’s Cyber and Information Security Research Group (CISR)
References
 [1] Christopher JC Burges. Dimension reduction: A guided tour. Now Publishers Inc, 2010.
 [2] Karl Pearson. Liii. on lines and planes of closest fit to systems of points in space. The London, Edinburgh, and Dublin Philosophical Magazine and Journal of Science, 2(11):559–572, 1901.

[3]
Sanjiv Kumar, Mehryar Mohri, and Ameet Talwalkar.
Sampling methods for the nyström method.
Journal of Machine Learning Research
, 13(Apr):981–1006, 2012.  [4] G. Birkhoff and G.C. Rota. Ordinary differential equations. Blaisdell book in pure and applied mathematics. Blaisdell Pub. Co., 1969.
 [5] Paul G Constantine. Active subspaces: Emerging ideas for dimension reduction in parameter studies, volume 2. SIAM, 2015.

[6]
Paul G Constantine, Brian Zaharatos, and Mark Campanelli.
Discovering an active subspace in a singlediode solar cell model.
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, 8(56):264–273, 2015.