<Tr> <Td> hyperreal numbers the (set of) hyperreals non-standard analysis </Td> <Td> R means the set of hyperreal numbers . Other sets can be used in place of R . </Td> <Td> N is the hypernatural numbers . </Td> </Tr> <Tr> <Td> Hodge dual Hodge dual; Hodge star linear algebra </Td> <Td> ∗ v means the Hodge dual of a vector v. If v is a k - vector within an n - dimensional oriented inner product space, then ∗ v is an (n − k) - vector . </Td> <Td> If (e i) (\ displaystyle \ (e_ (i) \)) are the standard basis vectors of R 5 (\ displaystyle \ mathbb (R) ^ (5)), ∗ (e 1 ∧ e 2 ∧ e 3) = e 4 ∧ e 5 (\ displaystyle * (e_ (1) \ wedge e_ (2) \ wedge e_ (3)) = e_ (4) \ wedge e_ (5)) </Td> </Tr> <Tr> <Td> Kleene star Kleene star computer science, mathematical logic </Td> <Td> Corresponds to the usage of * in regular expressions . If ∑ is a set of strings, then ∑ * is the set of all strings that can be created by concatenating members of ∑ . The same string can be used multiple times, and the empty string is also a member of ∑ * . </Td> <Td> If ∑ = (' a',' b',' c') then ∑ * includes' ',' a',' ab',' aba',' abac', etc . The full set cannot be enumerated here since it is countably infinite, but each individual string must have finite length . </Td> </Tr> <Tr> <Td> ∝ </Td> <Td> ∝ (\ displaystyle \ propto \! \,) </Td> <Td> proportionality is proportional to; varies as everywhere </Td> <Td> y ∝ x means that y = kx for some constant k . </Td> <Td> if y = 2x, then y ∝ x . </Td> </Tr>

What is the meaning of x in algebra