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I have submitted my mathematical paper to a journal in November 2018. After that, it has the status "Awaiting Referee Selection" during 4 months. I contacted the Editor-in-Chief for his support, luckily, he is very nice. The result is that the status of the paper has been "under review" in March 2019 and he said that:
I hope to receive the reports of the referees by mid-April.
However, today (April 24, 2019) I still has not received the first decision. Should I contact the Editor-in-Chief again or not? Thank you for your comments.