function delta = ydelta(y)
% This function finds the delta-circuit of resistors on three 
% vertices which is equivalent to an inputed y-circuit on the 
% same three vertices with a central fourth vertex added. 
%
% INPUT: resistances on the top, left, and right edges in a circuit
% like this:
%
%    |
%   / \
%
% OUTPUT: the bottom, left and right resistances on a circuit like
% this:
% 
%   /\
%  /__\
%

sigma = y(1)*y(2)+y(2)*y(3)+y(1)*y(3) ;

delta(1) = sigma / y(1) ;
delta(2) = sigma / y(2) ;
delta(3) = sigma / y(3) ;