<Tr> <Td> quotient set mod set theory </Td> <Td> A / ~ means the set of all ~ equivalence classes in A . </Td> <Td> If we define ~ by x ~ y ⇔ x − y ∈ Z, then R / ~ = (x + n: n ∈ Z, x ∈ (0, 1)). </Td> </Tr> <Tr> <Td> √ </Td> <Td> √ (\ displaystyle \ surd) \ surd x (\ displaystyle (\ sqrt (x))) \ sqrt (x) </Td> <Td> square root (radical symbol) the (principal) square root of real numbers </Td> <Td> √ x means the nonnegative number whose square is x . </Td> <Td> √ 4 = 2 </Td> </Tr> <Tr> <Td> complex square root the (complex) square root of complex numbers </Td> <Td> If z = r exp (iφ) is represented in polar coordinates with − π <φ ≤ π, then √ z = √ r exp (iφ / 2). </Td> <Td> √ − 1 = i </Td> </Tr> <Tr> <Td> ∑ </Td> <Td> ∑ (\ displaystyle \ sum) \ sum </Td> <Td> summation sum over...from...to...of calculus </Td> <Td> ∑ k = 1 n a k (\ displaystyle \ sum _ (k = 1) ^ (n) (a_ (k))) means a 1 + a 2 + ⋯ + a n (\ displaystyle a_ (1) + a_ (2) + \ cdots + a_ (n)). </Td> <Td> ∑ k = 1 4 k 2 = 1 2 + 2 2 + 3 2 + 4 2 = 1 + 4 + 9 + 16 = 30 (\ displaystyle \ sum _ (k = 1) ^ (4) (k ^ (2)) = 1 ^ (2) + 2 ^ (2) + 3 ^ (2) + 4 ^ (2) = 1 + 4 + 9 + 16 = 30) </Td> </Tr>

What is the meaning of x and y in math