Perhaps you should try some spelling variations:
"call options research : july 2013" » "call actions research : july 2013", "call optics research : july 2013", "call locations research : july 2013", "call option research : july 2013", "real options research : july 2013"
"call operations research : july 2007" » "rail operations research : july 2007", "chain operations research : july 2007", "cargo operations research : july 2007", "call operations research : july 2008", "call operations research : july 2012", "call operations research : july 2020"

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

• Performing a fuzzy search may retrieve terms with similar spellings: (("call options research : july 2013") or ("call operations research : july 2007"))~.
• If you are trying to use Boolean operators, they must be ALL CAPS: (("call options research : july 2013") OR ("call operations research : july 2007")).
• Removing quotes may allow a broader search: (( call options research : july 2013 ) or ( call operations research : july 2007 )).
• Adding a wildcard symbol may retrieve word variants: (("call options research : july 2013") or ("call operations research : july 2007"))*.