What does  mean in Relational Calculus? I saw a Tuple Relational Calculus formula, and it contained the symbol &lt;&gt;

I couldn't find any reference to it anywhere - can anyone tell me what this means?
(perhaps this question will be a future first Google result)
&lt;> is not defined in relational calculus (or anywhere in math). It however is often used in programming languages (like SQL) to express "not equal". The correct sign would be the equal sign with a vertical line through its center. (= and | on top of each other).