Time independent vs. time dependent lagrange multiplier

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What are the differences between these two in applications? For example: $$max\sum_{t=0}^{\infty} \beta^t u(c_t)$$$$s.t.f(c_t,c_{t+1},x_t,x_{t+1})=0$$ What are the differences between: $$L=\sum_{t=0}^{\infty} \beta^t u(c_t)+\lambda_tf(c_t,c_{t+1},x_t,x_{t+1})$$ and $$L=\sum_{t=0}^{\infty} \beta^t u(c_t)+\lambda f(c_t,c_{t+1},x_t,x_{t+1})$$ My prof says it represents the same problem... But in some cases I do find it gives different results...