In mathematics, specifically in the field of group theory, the McKay conjecture is a conjecture of equality between the number of irreducible complex characters of degree not divisible by a prime number ${\displaystyle p}$ to that of the normalizer of a Sylow ${\displaystyle p}$-subgroup. It is named after Canadian mathematician John McKay.

Suppose ${\displaystyle p}$ is a prime number, ${\displaystyle G}$ is a finite group, and ${\displaystyle P\leq G}$ is a Sylow ${\displaystyle p}$-subgroup. Define ${\displaystyle {\textrm {Irr}}_{p'}(G):=\{\chi \in {\textrm {Irr}}(G):p\nmid \chi (1)\}}$ where ${\displaystyle {\textrm {Irr}}(G)}$ denotes the set of complex irreducible characters of the group ${\displaystyle G}$. The McKay conjecture claims the equality

${\displaystyle |{\textrm {Irr}}_{p'}(G)|=|{\textrm {Irr}}_{p'}(N_{G}(P))|}$

where ${\displaystyle N_{G}(P)}$ is the normalizer of ${\displaystyle P}$ in ${\displaystyle G}$.

In McKay's original papers on the subject,^[1][2] the statement was given for the prime ${\displaystyle p=2}$ and simple groups, but examples of computations of ${\displaystyle |{\textrm {Irr}}_{p'}(G)|}$ for odd primes or symmetric groups are mentioned. Marty Isaacs also checked the conjecture for the prime 2 and solvable groups ${\displaystyle G}$.^[3] The first appearance of the conjecture for arbitrary primes is in a paper by Jon L. Alperin giving also a version in block theory, now called the Alperin-McKay conjecture.^[4]

In 2007, Marty Isaacs, Gunter Malle and Gabriel Navarro showed that the McKay conjecture reduces to the checking of a so-called inductive McKay condition for each finite simple group.^[5][6] This opens the door to a proof of the conjecture by using the classification of finite simple groups.

The paper of Isaacs-Malle-Navarro was also an inspiration for similar reductions for Alperin weight conjecture, its block version, the Alperin-McKay conjecture and Dade's conjecture.

The McKay conjecture for the prime 2 was proven by Gunter Malle and Britta Späth in 2016.^[7]

An important step in proving the inductive McKay condition for all simple groups is to determine the action of the group of automorphisms ${\displaystyle {\textrm {Aut}}(G)}$ on the set ${\displaystyle {\textrm {Irr}}(G)}$ for each finite quasisimple group ${\displaystyle G}$. The solution has been announced by Späth^[8] in the form of an ${\displaystyle {\textrm {Aut}}(G)}$-equivariant Jordan decomposition of characters for finite quasisimple groups of Lie type.

The McKay conjecture for all primes and all finite groups was announced by Marc Cabanes and Britta Späth in October 2023 in various conferences, a manuscript being available later in 2024.^[9]