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Note 10 - Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2020
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
   
Number of
    
Weighted
  
   
Common Stock
    
Average
  
   
Warrants
    
Exercise Price
  
Balance, December 31, 2019
    
2,435,807
     $
6.36
  
Warrants for common stock sold
    
—
      
—
  
Warrants for common stock exercised
    
(11,083
 )    
6.60
  
Balance, December 31, 2020
    
2,424,724
     $
6.36