While learning time-independent perturbation theory, there was something that I couldn't understand, and it has to do something with order of $\lambda\\$
For example, when deriving the equation for time-independent perturbation of energy eigenstates, we assume that the hamiltonian can be expressed as a combination of the unperturbed and perturbed hamiltonians. i.e. $H=H_0 +\lambda\ H_1 $. Along with that equation, there comes this condition that $H'=\lambda\ H_1$ and that $\lambda\ <<1$ (which does seem to make sense. I mean, you would want the perturbing part to be small)
However, some problems arise when I also did the similar things to eigen states. The author assumes that
$|n>=|n^0 > +\lambda\ |n^1> + \lambda\ ^2 |n^2> + \cdots$
this (somewhat) seems to make sense. I mean, if we can expand almost any function using the Taylor expansion, why not do it to states too?
But what I can't understand is when we put those two together in the Energy Eigenvalue equation.
$(H_0 +\lambda\ H_1)(|n^0 > +\lambda\ |n^1> + \lambda\ ^2 |n^2> + \cdots)=(E_n^1+\lambda E_n^2 +\cdots )(|n^0 > +\lambda\ |n^1> + \lambda\ ^2 |n^2> + \cdots)$
Then, we collect the terms in powers of $\lambda $ and then say that since the equation should be true no matter what value of $\lambda$ we give, the left side and the right side are equivalent, and therefore the coefficient of the $\lambda$ on the right side equals the coeffient of the same thing on the left side etc etc.
However, why does the equation have to hold for every $\lambda$? I can't seem to intuitively understand why.