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Note 6 - Stockholders' Equity and Stock-based Compensation - ESPP Assumptions (Details) - The 2017 Employee Stock Purchase Plan [Member]
3 Months Ended
Mar. 31, 2024
Mar. 31, 2023
Expected volatility 98.00% 83.00%
Dividend yield 0.00% 0.00%
Minimum [Member]    
Expected term in years (Year) 6 months 6 months
Risk-free interest rate 4.90% 5.10%
Maximum [Member]    
Expected term in years (Year) 1 year 1 year
Risk-free interest rate 5.30% 5.20%