Memory Minimization

As I grow older, I’m starting to forget things (such as all the math I ever learned) … but that’s not the reason for the title of this post. A somewhat interesting question popped up on Mathematics StackExchange. It combines a basic sequencing problem (ordering the processing of computational tasks) with a single resource constraint …

More

Premature Obituaries

[T]he report of my death was an exaggeration. (Mark Twain, 1897) In a recent blog post, “Data Science Is Not Dead“, Jean-Francois Puget discussed and dissented with a post by Jeroen ter Heerdt titled “Data Science is dead.” Barring the possibility that Schroedinger shoved data science into a box and sealed it, both assertions cannot …

More

If This and That Then Whatever

I was asked a question that reduced to the following: if \(x\), \(y\) and \(z\) are all binary variables, how do we handle (in an integer programming model) the requirement “if \(x=1\) and \(y=1\) then \(z=1\)”? In the absence of any constraints on \(z\) when the antecedent is not true, this is very easy: add …

More

Update Error: Wrong Architecture

Yesterday I ran into one of those mystifying glitches that, will infrequent, serve as a reminder that Linux is not for the faint of heart. When I booted my desktop system (Linux Mint 18.1 Serena), the system tray icon for the software updater was displaying a red “X” that indicates it tried and failed to …

More

Adventures with TeX Live

Since I use the Linux Mint operating system, the obvious (if not only) choice for a LaTeX distribution is TeX Live. (If you are not familiar with, or are not interested in, the LaTeX typesetting system, you have already read too far in this post.) On Mint, Ubuntu and other Debian-type operating systems, you typically …

More

MSU Students Do Microfinance

Several years ago, I found out about Kiva.org, an online “microfinance” site where individuals can make small loans ($25 is the standard increment at Kiva) to entrepreneurs in low income settings. The entrepreneurs actually apply for larger amounts, which they typically receive from third-party “field partners”. The entrepreneurs repay the loans with interest to the …

More

Another Absolute Value Question

Someone asked whether it is possible to eliminate the absolute value from the constraint \(Lx\le |y| \le Ux\) (where \(L\ge 0\) and \(U>0\) are constants, \(x\) is a binary variable, and \(y\) is a continuous variable). The answer is yes, but what it takes depends on whether \(L=0\) or \(L>0\). The easy case is when …

More

Fischetti on Benders Decomposition

I just came across slides for a presentation that Matteo Fischetti (University of Padova) gave at the Lunteren Conference on the Mathematics of Operations Research a few days ago. Matteo is both expert at and dare I say an advocate of Benders decomposition. I use Benders decomposition (or variants of it) rather extensively in my …

More

Pro Bono Analytics Is Growing Social

Pro Bono Analytics is a program by INFORMS (the Institute for Operations Research and the Management Sciences, for the acronym-averse), “the largest society in the world for professionals in the field of operations research (O.R.), management science, and analytics”. PBA “connects our members and other analytics professionals with nonprofit organizations working in underserved and developing communities”. …

More

Mapping Trackball Buttons

For years, I’ve used a Logitech M570 wireless trackball with my Linux Mint PC. I generally prefer trackballs to mice — no need to lift and reposition after a bunch of movement — and I find that using my thumb, rather than my index finger (or, if I’m in a bad mood, my middle finger) …

More

Mint Upgrade Woes

As I mentioned a couple of months ago, I upgraded a laptop from Linux Mint 17.3 (“Rebecca”) to 18.0 (“Sarah”) with minimal difficulties. My laptop serves as a guinea pig for these things. Once I’m sure things work fine on the laptop, I’ll consider upgrading my desktop as well. A few days ago I finally …

More

MIP Models in R with OMPR

A number of R libraries now exist for formulating and solving various types of mathematical programs in R (or formulating them in R and solving them with external solvers). For a comprehensive listing, see the Optimization and Mathematical Programming task view on CRAN. I decided to experiment with Dirk Schumacher’s OMPR package for R. OMPR …

More

Surviving an nVidia Driver Update

Scenario: I’m running Linux Mint 17.3 Rebecca (based on Ubuntu 14.04) on a PC with a GeForce 6150SE nForce 430 graphics card. My desktop environment is Cinnamon. The graphics card is a bit long in the tooth, but it’s been running fine with the supported nVidia proprietary driver for quite some time. Unfortunately, having no …

More

Mint Upgrade Problem Solved

I decided to upgrade first my laptop (the canary in the mine shaft), then my desktop (assuming the canary lived), from Linux Mint 17.3 (“Rebecca”) to 18.0 (“Sarah”). The “old school” approach would be to download the distribution to a CD/DVD, or create a bootable version on a USB stick, test it and then install …

More

DNS on Home WiFi

DNS on Home WiFi Like a lot of people, I have a home WiFi network, and I recently changed my Internet service provider. That required replacing my cable modem. Both the old and new modems provided a DHCP service that doled out local IP addresses to devices connecting to the home WiFi network. Both modems …

More

Better Estimate, Worse Result

I thought I might use a few graphs to help explain an answer I posted on Quora recently. First, I’ll repeat the question here: In parametric optimization with unknown parameters, does a better estimator of the parameter always lead to better solutions to the actual problem? Here we’re trying to minimize f(x,θ) with respect to …

More

Hovering Over Shiny Plots

I’m following up on yesterday’s post, “Formatting in a Shiny App“. One of the features I added to my Shiny application was the ability to identify a point in a plot by hovering over it. Since I wanted to do this in several different plots, and did not want to reproduce the logic each time, …

More

Formatting in a Shiny App

I’ve been updating a Shiny (web-based interactive R) application, during the course of which I needed to make a couple of cosmetic fixes. Both proved to be oddly difficult. Extensive use of Google (I think I melted one of their cloud servers) eventually turned up enough clues to get both done. I’m going to record …

More

Some R Resources

(Should I have spelled the last word in the title “ResouRces” or “resouRces”? The R community has a bit of a fascination about capitalizing the letter “r” as often as possible.) Anyway, getting down to business, I thought I’d post links to a few resources related to the R statistical language/system/ecology that I think may …

More

Connecting Bluetooth Headphones

I recently picked up a pair of Bluetooth headphones (Mixcder ShareMe 7) for use with my laptop (which runs Linux Mint). Getting them to connect properly was a bit of an adventure. After I had things (mostly) sorted out, I decided to script the steps necessary to get them working so that I could just …

More

Finding the Kernel of a Matrix

I’m working on an optimization problem (coding in Java) in which, should various celestial bodies align the wrong way, I may need to compute the rank of a real matrix and, if it’s less than full rank, a basis for its kernel. (Actually, I could get by with just one nonzero vector in the kernel, …

More

Over- and Underfitting

I just read a nice post by Jean-François Puget, suitable for readers not terribly familiar with the subject, on overfitting in machine learning. I was going to leave a comment mentioning a couple of things, and then decided that with minimal padding I could make it long enough to be a blog post. I agree …

More

Enforcing Simultaneous Arrivals

I’m recapping here an answer to a modeling question that I just posted on a help forum. (Since it will now appear in two different places, let’s hope it’s correct!) The original poster (OP) was working on a routing model, in which vehicles (for which I will use and if needed as indices) are assigned …

More

MathJax Whiplash

Technology is continuously evolving, and for the most part that’s good. Every now and then, though, the evolution starts to look like a random mutation … the kind that results in an apocalyptic virus, or mutants with superpowers, or something else that is much more appealing as a plot device in a movie or TV …

More

Using CLP with Java

The COIN-OR project provides a home to a number of open source software projects useful in operations research, primarily optimization programs and libraries. Possibly the most “senior” of these projects is CLP, a single-threaded linear program solver. Quoting the project description: CLP is a high quality open-source LP solver. Its main strengths are its Dual …

More

Java “Deep Learning” Library

If you are a Java (or Scala) (or maybe Clojure?) programmer interested in analytics, and in particular machine learning, you should take a look at Deeplearning4j (DL4J). Quoting their web site: Deeplearning4j is the first commercial-grade, open-source, distributed deep-learning library written for Java and Scala. Integrated with Hadoop and Spark, DL4J is designed to be …

More

Accessing R Objects By Name

At a recent R user group meeting, the discussion at one point focused on two of the possibly lesser known (or lesser appreciated?) functions in the base package: get and assign. The former takes a string argument and fetches the object whose name is contained in the string. The latter does the opposite, assigning an …

More

SOLVED: Problem with Impressive

I’ve written before (here and here) about using the open-source Impressive program to display PDF presentations. It’s been quite a while since I used it, or any other presentation software for that matter — retired geezers don’t do a lot of presenting — but I’ll be helping the INFORMS Student Chapter at the University of …

More

Tale of the Loose Cable

Friday I used my laptop (Acer Aspire V5-131, Linux Mint 17.3) at a coffee shop with no problems. Saturday it wouldn’t boot. The specific error message was: Broadcom UNDI PXE2.1 v15.0.1 <blah blah blah> PXE-E61: Media test failure, check cable PXE-M0F: Exiting Broadcom PXE ROM No bootable device — insert boot disk and press any …

More

Coin Flipping

I don’t recall the details, but in a group conversation recently someone brought up the fact that if you flip a fair coin repeatedly until you encounter a particular pattern, the expected number of tosses needed to get HH is greater than the expected number to get HT (H and T denoting head and tail …

More

Oracle Java 8 Update

For quite a while, I was getting security nags from Firefox every time a web site wanted to run a Java applet. Firefox would tell me I needed to upgrade to the latest version of Java. That would have been fine, except that I was already running the latest Java (1.8.0_66 as of this writing). …

More

On Statistics, Reporting and Bacon

I’ve previously ranted about the need for a “journalistic analytics” college major, to help with reporting (and editing) news containing statistical analysis. Today I read an otherwise well written article that inadvertently demonstrates how easy it is for even seasoned reporters to slip up. The cover story of the November 9 issue of Time magazine, …

More

Regression Via Pseudoinverse

In my last post (OLS Oddities), I mentioned that OLS linear regression could be done with multicollinear data using the Moore-Penrose pseudoinverse. I want to tidy up one small loose end. Specifically, let be the matrix of predictor observations (including a column of ones if a constant term is desired), let be a vector of …

More

OLS Oddities

During a couple of the lectures in the Machine Learning MOOC offered by Prof. Andrew Ng of Stanford University, I came across two statements about ordinary least squares linear regression (henceforth OLS) that surprised me. Given that I taught regression for years, I was surprised that I could be surprised (meta-surprised?), but these two facts …

More

Producing Reproducible R Code

A tip in the Google+ Statistics and R community led me to the reprex package for R. Quoting the author (Professor Jennifer Bryan, University of British Columbia), the purpose of reprex is to [r]ender reproducible example code to Markdown suitable for use in code-oriented websites, such as StackOverflow.com or GitHub. Much has been written about …

More

Another OR Educational Resource

Two years ago (two years and one day if you’re being picky), I posted a pointer to a Spanish language web site hosted by Francisco Yuraszeck (professor at the Universidad Técnica Federico Santa María in Viña del Mar, Chile). The site, Gestión de Operaciones, is listed in the resources box on the right. Recently, Francisco …

More

More Shiny Hacks

In a previous entry, I posted code for hack I came up with to add vertical scrolling to the sidebar of a web-based application I’m developing in Shiny (using shinydashboard). Since then, I’ve bumped into two more issues, leading to two more hacks that I’ll describe here. First, I should point out that I’m using …

More

Optimizing Part of the Objective Function

A somewhat curious question showed up on a forum today. The author of the question has an optimization model (I’ll assume it is either a linear program or mixed integer linear program) of the form \begin{alignat*}{2} & \textrm{maximize} & & \sum_{i=1}^{N}x_{i}\\ & \textrm{s.t.} & & x\in\mathcal{X} \end{alignat*} where the feasible region $\mathcal{X}$ is presumably polyhedral. …

More

Autocorrupt in R

You know that “autocomplete” feature on your smart phone or tablet that occasionally (or, in my case, frequently) turns into an “autocorrupt” feature? I just ran into it in an R script. I wrote a web-based application for a colleague that lets students upload data, run a regression, ponder various outputs and, if they wish, …

More

ORiginals – Videos About Research

ORiginals is a YouTube channel co-hosted by Dr. Banafsheh Behzad (@banafsheh_b) of CSU Long Beach and my colleague Dr. David Morrison (@drmorr0). They present short (five or six minute) videos featuring researchers describing their research to a general (non-expert) audience. Their tag line is “Outstanding research in everyday language”, and I think the first two installments …

More

Shiny Hack: Vertical Scrollbar

I bumped into a scrolling issue while writing a web-based application in Shiny, using the shinydashboard package. Actually, there were two separate problems. The browser apparently cannot discern page height. In Firefox and Chrome, this resulted in vertical scrollbars that could scroll well beyond the bottom of a page. That’s mildly odd, but not a …

More