function y=double1(x)
%FUNCTION y=double1(x)
%  This imlements the doubling function from Devaney's book,
%  D(x).

y=2*x;
idx=find(x>=0.5);  %idx holds the indices of x whose
                   %entries are >= 1/2

y(idx)=y(idx)-1;   %Those entries in y get 1 subtracted