Sentence Correction : Need Explanation

[bhupen83]

Economist have long scratched their heads over this kind of pattern. One explanation is that this behavior emerges when two competing companies change their pricing strategy at each stage by reacting to the other.

In the above sentence is the use of 'have' is correct ? or otherwise, the word 'Economist' should be Economists' .

[aman88]

Correct ones:
Economists HAVE
OR
The economist has

Incorrect ones:
Economist have
Economists has

In this sentence, the pronoun THEIR identifies that the subject is plural -> 'Economists'

Rule: plural subject agrees with plural verb AND singular subject agrees with singular verb.
visit https://grammar.ccc.commnet.edu/grammar/sv_agr.htm for detailed explanation on subject-verb agreement
Thanks.

[viveksingh222]

Economist have long scratched their heads over this kind of pattern. One explanation is that this behavior emerges when two competing companies change their pricing strategy at each stage by reacting to the other.

In the above sentence is the use of 'have' is correct ? or otherwise, the word 'Economist' should be Economists' .

Take the pronoun form of the noun and compare..
He, she, it are followed by has .
I, we, you, they are followed by have.
Economist(He/She) has
Economists(They) have