Perhaps you should try some spelling variations:
a robert » gal robert
robert e » robert _, robert j
a robust » _ robust
hellman » helpman
heilmann » heinemann, hermann, heymann
book » books, ebook
heimann » hermann, heinemann
heitmann » heinemann, hermann, hartmann

You may be able to get more results by adjusting your search query.

• Performing a fuzzy search may retrieve terms with similar spellings: (( a robert e. (hellman or heilmann) book ) or ( a robust _. (heimann or heitmann) book ))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (( a robert e. (hellman OR heilmann) book ) OR ( a robust _. (heimann OR heitmann) book )).
• Adding a wildcard symbol may retrieve word variants: (( a robert e. (hellman or heilmann) book ) or ( a robust _. (heimann or heitmann) book ))*.