function G = givens(v)
% G=GIVENS(v) constructs the real Givens matrix which eliminates the  
% second component of the vector v of length 2
% v: a vector of length 2
% G: Givens matrix
s = 0;
c = 1;
if  abs(v(2)) < abs(v(1)) 
  tau = v(2)/v(1);
  c = 1/sqrt(1+tau^2);
  s = tau*c;
end
if abs(v(1)) < abs(v(2))
  tau = v(1)/v(2);
  s = 1/sqrt(1+tau^2);
  c = tau*s;
end
G = [c s;-s c];