Since "of bank robberies" is a prepositional phrase, neither the phrase nor any word in it can be the subject. Therefore, the subject is "series" -- and the word "a" in front of it indicates it's in singular form.
Which of those verbs is also in singular form?
http://grammar.ccc.commnet.edu/grammar/sv_agr.htm
a series of of bank robberies (have , has) occurred during the past several months
3 answers
Mrs. Smith need a complete evaluation for her complaint of pain.
His followed the care plan exactly and felt better.
1 answer