Below you find my personal collection links, scripts, guides and graphics you might find neat, or useful.

• Link'O'Rama
• Personal credo
• Gnuplot extrapolations with errorbars
• arXiv logo
• How to fix your citation index
• RSS-feed of PRL's newly accepted and cond-mat with authors
• MathJax
• Guide to migrate from Linux to MacOSX
• Condensed matter job sites
• Word count of TeX manuscript for Phys. Rev. Lett.
• Shell script to create histograms
• Honeycomb lattice graphics
• YouTube science
• Shell script grepskip
• Setup your Raspberry Pi as an AirPrint server
• Calling C/C++ routines from Fortran
• Dudeney-dissection

Histograms in the shell & gnuplot

This is a nice shell script (author unknown) which uses awk to produce a histogram from a data set. This can be especially useful in combination with gnuplot, e.g., for the original data set data.dat use the line

gnuplot> p "< histogram data.dat 100" u 1:2 w boxes

in gnuplot to plot the histogram of data.dat with 100 subdivisions (bins) of the whole data range. Unfortunately, the script's origin is lost - I apologize for not being able to crediting the original author. You can download the script here: histogram (make sure to put it in a directory in your $PATH and make it executable).

                  #!/bin/bash

                  FILE=$1 # input file
                  BINS=$2 # number of bins

                  if [ "$#" != "2" ] || [ ! -f $FILE ]; then
                     echo 'usage: histogram <file> <#bins>'
                     exit 1
                  fi

                  # determine border values
                  MIN=$(awk 'NR == 1 || $1 < min {line = $0; min = $1} END {print line}' $FILE)
                  MAX=$(awk 'NR == 1 || $1 > max {line = $0; max = $1} END {print line}' $FILE)

                  # use awk to bin data
                  awk -v m=$MIN -v M=$MAX -v b=$BINS 'BEGIN { 

                    w = (M-m)/b      # width of each bin

                    # set up arrays
                    for (i = 1 ; i <= b; ++i) {
                      n[i] = m+(i*w)        # upper bound of bin
                      c[i] = n[i] - (w/2)   # centre of bin
                      f[i] = 0              # frequency count
                    }
                  }

                  { 
                    # bin the data
                    for (i = 1; i <= b; ++i)
                      if ($1 <= n[i]) {
                           ++f[i]
                           break       
                      } 
                  }

                  END {
                    # print results to a file for plotting

                    for (i = 1; i <= b; ++i) {
                      if (f[i] > 0) {
                        print c[i],"\t", f[i]
                      }
                      else {
                        print c[i], 0
                      }
                    }
                  }' $FILE
               

Note, that gnuplot of course can create histograms intrinsically using the commands

gnuplot> binwidth=5
gnuplot> bin(x,width)=width*floor(x/width)
gnuplot> p 'data.dat' u (bin($1,binwidth)):(1.0) smooth freq with boxes

Setup your Raspberry Pi as an AirPrint server

Here are the step-by-step instructions on how to setup your Raspberry Pi running Raspbian (or any other debian system) as an AirPrint server. I am essentially reproducing the HOWTOs found here and here. First of all, let's install cups (if not already installed):

$ sudo apt-get install cups

Add your user (default user name is pi) to the group lpadmin to allow it administrate CUPS.

$ sudo usermod -a -G lpadmin pi

We also need to tell CUPS to alias itself to any hostname as AirPrint communicates with CUPS with a different hostname than the machine-defined one. To do this, we edit the CUPS configuration file and add the directive ServerAlias * before the <Location /> block. (You might want to use vi and do it manually, instead).

$ sudo sed -i -e 's/<Location \/>/ServerAlias *\n<Location \/>/' /etc/cups/cupsd.conf

Connect (as user pi, not root!) to the CUPS web-interface using midori (or better iceweasel)

$ midori https://localhost:631

and setup the new printer. You'll need to authenticate with your systems username and password. Make sure you check 'Share This Printer' during the printer setup! Note, there is no need to set 'Share printers' in the Server Settings: section on CUPS's Administration page. With the aid of Timothy J. Fontaine's python script we can now generate avahi announcements that will make iOS think that our printer is AirPrint capable. For that you’ll need the Python CUPS module and the avahi daemon (again we'll do this in a root-shell):

$ sudo apt-get install python-cups avahi-daemon
$ sudo /etc/init.d/avahi-daemon start

Now, we’ll download Tim’s Python script to generate the avahi announcements for all installed CUPS printers and restart the daemon

$ sudo mkdir /opt/airprint
$ sudo cd /opt/airprint
$ sudo wget -O airprint-generate.py --no-check-certificate \
     https://raw.github.com/tjfontaine/airprint-generate/master/airprint-generate.py
$ sudo chmod +x airprint-generate.py
$ sudo ./airprint-generate.py -d /etc/avahi/services
$ sudo /etc/init.d/avahi-daemon restart

And that's all there is to do. As long as your iOS device is in the same subnet you should now find the AirPrint representatives of your installed printers.

Dudeney dissection

A swing-hinged dissection is a kind of geometric dissection in which all of the pieces are connected into a chain by "hinged" points, such that the rearrangement from one figure to another can be carried out by swinging the chain continuously, without severing any of the connections.

The puzzle known as “Dudeney’s dissection” or the “haberdasher’s problem” considers the transformation from an equilateral triangle to a square. More than 122 years after it was first proposed, Kamata and colleagues have finally proved in December 2024 that a solution with fewer pieces is impossible, i.e., four-pieces are optimal. More details of the story can be found at scientificamerican.com.

While you can find many constructions to create your own version, they mostly only approximate the correct solution. It turns out that the sections x and y are not equal to 1/4, as often portrait in geometric constructions. They are close, though - here is a Mathematica notebook to convince yourself!