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Note 9 - Stockholders' Equity (Tables)
 12 Months Ended
Dec. 31, 2019
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
   
Number of
Common Stock
Warrants
    
Weighted
Average
Exercise Price
  
Balance, December 31, 2018
    
296,296
     $
6.75
  
Warrants for common stock sold
    
2,139,512
     $
6.30
  
Warrants for common stock exercised
    
-
      
 
  
Balance, December 31, 2019
    
2,435,808
     $
6.36