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Note 9 - Stockholders' Equity (Tables)
3 Months Ended
Mar. 31, 2022
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
  

Outstanding Warrants to Purchase Shares

  

Exercise Price Per Share

 

Expiration date

May 2018 warrants

  

762

  

$

4.05

 

May 30, 2022

December 2020 warrants

  

6,490

  

$

1.00

 

December 11 2024-June 21, 2025

January 2021 warrants

  

4,500

  

$

1.055

 

July 8, 2025

March 2021 warrants

  

10,525

  

$

2.88

 

September 22, 2025

   

22,277