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Accounts Receivable (Details) - USD ($)
Mar. 31, 2019
Dec. 31, 2018
Components of accounts receivable    
Accounts receivable $ 14,436,421 $ 13,855,040
Less: allowance for doubtful accounts (11,305,983) (9,527,060)
Accounts receivable, net $ 3,130,438 $ 4,327,980