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Note 3 - Basis of Presentation and Summary of Significant Accounting Policies (Tables)
6 Months Ended
Jun. 30, 2021
Notes Tables  
Schedule of Antidilutive Securities Excluded from Computation of Earnings Per Share [Table Text Block]
  

As of June 30,

 
  

2021

  

2020

 

Common stock warrants

  6,499,469   9,117,209 

Stock options

  3,052,887   1,427,829 

Unvested restricted stock awards

  153,000   98,100