Some databases and Internet search tools such as Google and ACM Digital Library use mathematical operators to combine search terms.
Mathematical operators are the plus symbol (+) and the minus symbol (-).
The plus symbol (+) in front of a term (no spaces) requires that term to be in the returned record and is therefore similar in concept to the AND operator.
The minus symbol (-) in front of a term requires that word to be excluded from the search results and therefore similar in concept to the NOT operator.
For example:
(+energy +solar) retrieve records containing both terms.
(energy - solar) retrieve records containing energy and excluding solar.