function Y = influence3d(x, m, k, df, N);

% influence(x, m, k, df, N) -- Compute influence based on distance factor
%
% The influence() function is deisgned to compute the influence of the xth
% nearest neighbor to a fish, given that there are N fish in the school,
% the maximum influence possible is m (usually 1), the "influence constant"
% is k, and the distance factor (usually computed by a different function)
% is df. The only required argument is x. Default values are m = 1, k = 0.1, 
% df = 1/2, and N = 16 fish. The influence function returns a fractional 
% number, by which the A/R function's result can then be multiplied. The 
% functional form of influence is:
%
%      /            m             \
% Y = |  -----------------------   | 
%      \  1 + k^(-X + df * N )    /   
% 
% Example:
% ģInfl = influence(4, 1, .1, 1/8, 32)		% Computes the influence of 4th NN
% Infl =
% 
%     0.5000


Y = m / ( 1 +  k^( - (x - ( df * N )) ) );