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The Kadison–Singer Problem
open
Posed by Richard Kadison / Isadore Singer · 1959 · operator algebras / combinatorics
The problem
In its Weaver form: every partition-compatible assignment of norms to vectors in \(R^{n}\) admits a partition into two classes each with small operator norm — a balanced-sparse decomposition statement equivalent to Kadison and Singer's extension question.
History & significance
Anderson recognised the difficulty early; Weaver's 2004 discrepancy reformulation gave the beachhead. Marcus, Spielman and Srivastava proved it in 2013 ('interlacing polynomials II'), via expected characteristic polynomials of random vectors — a technique so fertile it next yielded existence of bipartite Ramanujan graphs for every degree (solving a 25-year-old Lubotzky question) and improved smoothing bounds elsewhere. Published in Annals; awarded the 2014 Pólya Prize. The rare solve that opened three fields at once.
Still open.
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