What is "q-construction"

In algebra, Quillen's Q-construction associates to an exact category (e.g., an abelian category) an algebraic K-theory. More precisely, given an exact category C, the construction creates a topological space BC so that π(BC) is the Grothendieck group of C and, when C is the category of finitely generated projective modules over a ring R, for i = 0, 1, 2, π(BC) is the i-th K-group of R in the classical sense. (The notation "+" is meant to suggest the construction adds more to the classifying space BC.) One puts

K(C) = π(BC)
and call it the i-th K-group of C. Similarly, the i-th K-group of C with coefficients in a group G is defined as the homotopy group with coefficients:

K(C; G) = π(BC; G)
.

The construction is widely applicable and is used to define an algebraic K-theory in a non-classical context. For example, one can define equivariant K-theory as π of B of the category of equivariant sheaves on a scheme.

Waldhausen's S-construction generalizes the Q-construction in a stable sense; in fact, the former, which uses a more general Waldhausen category, produces a spectrum instead of a space. Grayson's binary complex also gives a construction of algebraic K-theory for exact categories. See also module spectrum#K-theory for a K-theory of a ring spectrum.