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Note 5 - Equity - Common Stock Purchase Warrant Activity (Details) - $ / shares
 9 Months Ended 12 Months Ended
Sep. 30, 2019
Dec. 31, 2018
Outstanding, number of warrants (in shares) 3,864,552 748,060
Outstanding, weighted average exercise price (in dollars per share) $ 4.60 $ 5.17
Issued, number of warrants (in shares) 0 3,166,492
Issued, weighted average exercise price (in dollars per share) $ 4.47
Cancelled or expired, number of warrants (in shares) (15,385)
Exercised, number of warrants (in shares) (34,615)
Outstanding, number of warrants (in shares) 3,864,552 3,864,552
Outstanding, weighted average exercise price (in dollars per share) $ 4.60 $ 4.60