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Note 7 - Stockholders' Equity, Stock Options and Warrants - Summary of Transactions for Stock Options and Warrants (Details) - $ / shares
3 Months Ended 12 Months Ended
Mar. 31, 2020
Dec. 31, 2019
Warrant [Member]    
Number of Shares Outstanding (in shares) 2,171,610 362,664
Average Exercise Price Outstanding (in dollars per share) $ 15.26 $ 41.67
Number of Shares Issued (in shares) 5,001,591 1,869,299
Average Exercise Price Issued (in dollars per share) $ 1.42 $ 9.25
Number of Shares Forfeited (in shares) (119,441) (653)
Average Exercise Price Forfeited (in dollars per share) $ 6.47 $ 3,249.28
Number of Shares Exercised (in shares) (59,700)
Average Exercise Price Exercised (in dollars per share) $ 0.10
Number of Shares Outstanding (in shares) 7,053,760 2,171,610
Average Exercise Price Outstanding (in dollars per share) $ 4.57 $ 15.26
Share-based Payment Arrangement, Option [Member]    
Number of Shares Outstanding (in shares) 766,424 366,928
Average Exercise Price Outstanding (in dollars per share) $ 11.34 $ 17.03
Number of Shares Issued (in shares) 38,350 423,295
Average Exercise Price Issued (in dollars per share) $ 2.37 $ 6.53
Number of Shares Forfeited (in shares) (10,736) (23,799)
Average Exercise Price Forfeited (in dollars per share) $ 9.20 $ 13.30
Number of Shares Exercised (in shares)
Average Exercise Price Exercised (in dollars per share)
Average Exercise Price Exercised (in dollars per share)
Number of Shares Outstanding (in shares) 794,038 766,424
Average Exercise Price Outstanding (in dollars per share) $ 10.90 $ 11.34