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Note 4 - Warrants to Purchase Common Stock - Warrant Activity (Details) - $ / shares
 3 Months Ended 12 Months Ended
Mar. 31, 2021
Dec. 31, 2020
Dec. 31, 2019
Warrants, number of shares (in shares) 479,554 84,295  
Warrants, exercise price (in dollars per share) $ 0.97 $ 1.95  
Warrants, average remaining life (Year) 3 years 313 days   4 years 193 days
Issued, number of shares (in shares)    
Exercised, number of shares (in shares)   (115,385)  
Exercised, exercise price (in dollars per share)   $ 0.01  
Warrants, number of shares (in shares) 479,554 479,554 84,295
Warrants, exercise price (in dollars per share) $ 0.97 $ 0.97 $ 1.95
Minimum [Member]      
Issued, exercise price (in dollars per share)   0.01  
Maximum [Member]      
Issued, exercise price (in dollars per share)   $ 1.95