Annual report pursuant to Section 13 and 15(d)

Intangible assets, net - Amortization Expense (Details)

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Intangible assets, net - Amortization Expense (Details) - USD ($)
12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Intangible assets, net      
Amortization expense $ 967,000 $ 838,000 $ 622,000