function dNdt = expDec(t,N,tau)
%dNdt = expDec(t,N,tau);
% calculate dN/dt = -N/tau
%   t --> time
%   N --> number of nuclei
% tau --> e-folding time of decay
%dNdt <== value of derivative at time t
  
% 1st-order ode for exponential decay, with time constant tau
dNdt = -N/tau;

return