A = A + 1 is never a valid mathematical statement. Subtract A from both sides of the equation and you’re left with 0 = 1, which is never true. The single and double =’s used in certain programming languages are just syntactical operators, not mathematical statements.
The expression a = a + 1 simply instructs the microprocessor to increment some value a by 1. It does not mean that a actually equals one more than itself, which is a mathematical absurdity.
You’re right that it’s not a valid mathematical statement. In a programming class, it’s a valid assignment statement in many languages. The context was a math major in a programming class and I’m fairly certain that c19fan meant it as a joke.
10 for i=1 to 10 step 1
20 x=x*i
30 print i
40 next