# Double subscript error

$$f_s = \frac{1}{T} \geq 2 f_m_a_x$$


I am getting double subscript error. Although I am getting correct format of equation in pdf but in code it is giving this error. can anybody help me in solving that. Thank You

• Do you want to have max as one word ? use f_{max} – albert Nov 8 '18 at 10:12

\documentclass{article}
$$f_s = \frac{1}{T} \geq 2 f_{\mathrm{max}}$$

• Why \mathit? The most common usage is \mathrm. And braces should be added around the subscript; it works without them, but by chance. – egreg Nov 8 '18 at 10:48
• It's very possible that the model document has the wrong, but unfortunately very common f_{max}. Textual subscripts should always be upright. – egreg Nov 8 '18 at 11:01