Is copying, distributing, publicly performing, or publicly displaying a public domain work infringement?

No. Copyright infringement can occur only if copyright protection exists. A public domain work is not protected by copyright, so there is nothing to infringe. Therefore, copying, distributing, performing, or displaying a public domain work is not copyright infringement. Copying, distributing, performing or displaying a work that is a modification or a compilation of a public domain work or works, however, could be copyright infringement if the modification or compilation is protected by copyright.