nLab
Quillen plus construction

The Quillen plus construction is a method for simplifying the fundamental group of a space without changing its homology and cohomology groups. It was introduced by Kervaire (1969), but was crucially used Daniel Quillen to define the algebraic K-theory groups by applying it to the classifying space of the stable general linear group, $GL(A)$, of a ring $A$.

The plus construction doesn’t change the homology of the space but ‘kills’ a perfect normal subgroup of $G$ within the classifying space $BG$. For algebraic K-theory, that subgroup is the stable elementary linear group?, $E(A)$ within $GL(A)$.

References

• M. Kervaire,_Smooth homology spheres and their fundamental groups_, Trans. Amer. Math. Soc., 144 (1969) pp. 67–72

See section IV.1 of

• Charles Weibel, The K-Book: An introduction to algebraic K-theory (web)

See also

• Dustin Clausen?, Plus construction considerations, MO/121351/2503.