function res=etape(p,u);
N=2^p;
M=N/2;
u1=u(1:M);
u2=u(M+1:N);
res=zeros(u);
omega=exp(-2*%pi*%i/N);    	    
for j=1:M
   A=papillon([u1(j);u2(j)],omega^(j-1))
   res(j)=A(1);res(j+M)=A(2);
end;
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