In my Handbook of Writing for the Mathematical Sciences I described the spotlight factor, originally introduced by Tompa in 1989. The spotlight factor is defined for the first author of a paper in which there are 
The spotlight factor is the probability that if 
The smallest spotlight factor I have found is the value 0.0244 for Zielinski, for the paper
Pawel Zielinski and Krystyna Zietak, The Polar Decomposition—Properties, Applications and Algorithms, Applied Mathematics, Ann. Pol. Math. Soc. 38, 23-49, 1995
This beats the best factor of 0.0251 reported by Tompa in a 1990 follow-up paper.
Can you do better?
Here is a MATLAB M-file to compute the spotlight factor, preceded by an example of its usage:
>> spotlight('zielinski',1) ans = 2.4414e-02
function s = spotlight(x, k) %SPOTLIGHT Tompa's spotlight factor of authorship. % SPOTLIGHT(X, K) is the spotlight factor for the author whose % last name is specified in the string X, with K coauthors. % Mixed upper and lower case can be used. % Smaller spotlight factors correspond to rarer events. % Reference: % Martin Tompa, Figures of Merit, SIGACT News 20 (1), 62-71, 1989 if ~ischar(x), error('First argument must be a string.'), end if nargin < 2, error('Must give two arguments.'), end x = double(upper(x)) - double('A') + 1; x( find(x < 0 | x > 26) ) = 0; % Handle punctuation and spaces. s = 0; % Ideally use Horner's rule, but the following is clearer. for i=1:length(x) t = x(i); s = s + t/27^i; end s = (1 - s)^k;
Nick Higham 
Almost by a factor 10:
Zybarev, E.Yu.; Zybarev, Yu.M., “Petri nets as a language of specifications of discrete systems.”
0.002639
Python 3 version of the above code (comments removed for brevity):
def spotlight(x, k): if not isinstance(x, str): raise ValueError('First argument must be a string.') x = [ ord(c) - 64 if 'A' <= c <= 'Z' else 0 for c in x.upper() ] s = 0; for i in range(len(x)): t = x[i] s = s + t / 27**(i+1); return (1 - s)**k;Btw, shouldn’t spaces and punctuation be removed, and not just treated as zeroes?
That one will be hard to beat, Tompa states “blanks and punctuation represent zero”.
I don’t know of a paper with a smaller spotlight factor, but I do know of one or two MATLAB functions that make the code you posted a bit shorter. [Whether they make the code clearer or not is an open question.] The FLIP (or you could use FLIPLR) call is necessary in this approach since the higher powers of (1/27) are associated with the later letters in the last name. By flipping it, coeffs is now in the form POLYVAL expects. I checked this code with Zielinski and Zybarev and the results agreed with Nick and Vedran’s answers.
function spotFactor = spotlight(lastName, numCoauthors)
narginchk(2, 2);
assert(ischar(lastName), ‘First input must be a char array’);
nameNums = zeros(size(lastName));
findLetters = isletter(lastName);
nameNums(findLetters) = upper(lastName(findLetters))-‘A’+1;
coeffs = flip(nameNums, 2);
s = (1/27)*polyval(coeffs, (1/27));
spotFactor = (1-s)^numCoauthors;