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Note 9 - Stockholders' Equity (Tables)
9 Months Ended
Sep. 30, 2021
Notes Tables  
Schedule of Stockholders' Equity Note, Warrants or Rights [Table Text Block]
  

Outstanding Warrants to Purchase Shares

  

Exercise Price

 

Expiration date

May 2018 warrants

  762,344  $4.05 

May 30, 2022

December 2020 warrants

  6,489,500  $1.00 

December 11, 2024-June 21, 2025

January 2021 warrants

  4,500,000  $1.055 

July 8, 2025

March 2021 warrants

  10,525,000  $2.88 

September 22, 2025

   22,276,844