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Note 3 - Basis of Presentation and Summary of Significant Accounting Policies (Tables)
12 Months Ended
Dec. 31, 2021
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
  

December 31,

 
  

2021

  

2020

 

Common stock warrants

  6,499,469   9,100,112 

Stock options

  3,626,223   2,240,204 

Unvested restricted stock units

  275,450   153,000 
   10,401,142   11,493,316