let y, x1, x2 all be non-stationary I(1) variables:

uder what conditions is using a single-equation approach to testing for cointegration valid in that it provides an efficient estimate of the long-run equilibrium relationship between y, x1 and x2?

即是：y,x1和x2都是可一阶单整的不稳定变量，问在什么条件下用单方程检验协整在长期稳定的情况下对y,x1和x2的估计是有效估计？