400. That’s an error.

Google cannot redirect you to http://www.amazon.com/exec/obidos/external-search/?field-keywords=Briefwechsel+zwischen+C.F.+Gauss+H.C.+Schumacher,+herausg.+von+C.A.F.+Peters&index=books because the request was invalid.

This might be because the link was created a long time ago. Try your search again on Google. That’s all we know.