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Note 11 - Summary of Stock Options - Valuation Assumptions for Stock Options (Details)
3 Months Ended
Mar. 31, 2020
Mar. 31, 2019
Expected term (Year) 5 years 5 years
Average volatility 73.00% 73.00%
Risk-free interest rate 0.45% 2.53%
Dividend yield 0.00% 0.00%
Nonemployees [Member]    
Expected term (Year) 5 years 5 years
Average volatility 73.00% 73.00%
Risk-free interest rate 1.39% 2.49%
Dividend yield 0.00% 0.00%