Suppose I find a patent (A1) with claims 1, 2 and 3. Claim 2 is dependent from claim 1. If I find a prior reference that rejects claim 1. What happens with claim 2? Is still valid? Should they rearrange that claim?
A dependent claim is ALWAYS valid when its independent claim is valid. However, if an independent claim is invalidated, that DOES NOT ALWAYS necessarily mean that dependent claim is also invalid.
However, the procedure for amending the independent claim by including limitations from the dependent claim(s), and the process of examining the patentability of the amended claim can vary from one country to another.
If the document ends in A1 it represents the publication for an application, not a granted patent. If the application is still in the prosecution stage and this prior art was known the the examiner claim 1 would be rejected. The applicant could respond in a number of ways including amending claim to also include the limitations of claim 2 (and canceling claim 2). Or claim 1 could be amended in some other way that gets it past the reference. During prosecution a claim is not invalidated, just objected to and/or rejected. Invalidation is something a judge does in an environment (court) that doesn't allow for amending.
As a patent owner, you can leverage in enforcing patent against an infringer and negotiating or maintaining the terms of a license agreement by using the validity or invalidity of a patent claim as the basis.
Once issued, a patent is presumed valid, this is as mandated by the U.S. patent statutes. This means that each claim is presumed valid independently from any other claims. Which also means that even if the independent claim is considered invalid, the dependent claim is still valid and enforceable and would require a separate validity analysis.