This is the secret hack. COMSOL retains an archive of every version they have ever released. If you have a (a .lic file from a university or employer that paid for version 3.5a in 2006), support will give you the installer for free. If you don't have a license, ask your university library if they have an old site license.
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While the current version of COMSOL is far more powerful, version 3.5a is remembered for several reasons: comsol multiphysics 35a free download best