function g = tran_payoff_CON_put(xgrid,tau,k)
% TRAN_PAYOFF_CON_PUT - the transformed payoff for a cash-or-nothing put 
%   
% Written by:
% -- 
% John L. Weatherwax                2008-06-11
% 
% email: wax@alum.mit.edu
% 
% Please send comments and especially bug reports to the
% above email address.
% 
%-----

global b; 

% convert to a column vector: 
xgrid = xgrid(:); 
nx    = length(xgrid); 

% the transformed payoff: 
g = b * exp( 0.25*((k+1)^2)*tau ) * exp( 0.5*(k-1)*xgrid ) .* (xgrid < 0.0);