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Note 2 - Summary of Significant Accounting Policies (Tables)
9 Months Ended
Sep. 30, 2021
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
  

September 30,

 

Excluded potentially dilutive securities (1):

 

2021

  

2020

 
         

Options to purchase common stock

  1,038,439   1,390,340 

Unvested restricted stock units

  3,652,686   3,947,819 

Financing warrants to purchase common stock

  6,372   103,500 

Total potential dilutive securities

  4,697,497   5,441,659