Array indices must be positive integers or logical values. How to solve?

Answer 1

[Walter Roberson]

A3 (sigma3 / Ua)

is a request to either call a function named A3 passing in sigma3/Ua as a parameter, or else to index a variable named A3 at location sigma3/Ua .

If you compare to the rest of your code, what you probably want is

a3 * (sigma3 / Ua)

Answer 2

[Sulaymon Eshkabilov]

a1 =  0.22; a2 = a1; a3 = a2;
gamma1 = .194; gamma2 = gamma1; gamma3 = gamma1;
sigma1 =.1;  sigma2=sigma1; sigma3 = sigma1; 
MD = 1;
Ua = input('Enter Ua:  ');
n = (1-MD / (a1 * (sigma1 / Ua) ^ (- gamma1)) + a2 * (sigma2 / Ua) ^ (- gamma2) + a3*(sigma3 / Ua) ^ (- gamma3)) ^ 3;