def cwum_2d(x,c=0.):
    """
    Correlated density with uniform marginals (CWUM)
    x in [0,1], 0<=c<2
    c = 1. is tensor product uniform
    c = 0. # if c=0 max pdf(x) = 2. min= 0.
    """
    normalization = (13.*c+3)/16.
    return (c-2.*(c-1.)*((x[0,:]+1)/2.+(x[1,:]+1)/2.-2.*(x[0,:]+1)/2.*(x[1,:]+1)/2.)/4.)/normalization