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Note 2 - Significant Accounting Policies - Stock Option Valuation Assumptions (Details) - $ / shares
3 Months Ended
Mar. 31, 2024
Mar. 31, 2024
Mar. 31, 2023
Expected volatility   80.00% 82.00%
Weighted-average grant date fair value per share (in dollars per share)   $ 3.91 $ 3.84
Minimum [Member]      
Risk-free interest rate   3.80% 3.50%
Expected life (in years) (Year) 5 years 4 months 17 days   5 years 7 months 17 days
Maximum [Member]      
Risk-free interest rate 4.30%   4.20%
Expected life (in years) (Year) 6 years 3 months   6 years 3 months