*…while we keep discussing why mathematics is possible…*

## The news

Adam Marcus, Dan Spielman, and Nikhil Srivastava posted a paper entitled “Interlacing Families II: Mixed Characteristic Polynomials and the Kadison-Singer Problem,” where they prove the 1959 Kadison-Singer conjecture.

(We discussed part I of “interlacing families” in this post about new Ramanujan graphs. Looks like a nice series.)

## The Kadison-Singer Conjecture

The Kadison-Singer conjecture refers to a positive answer to a question posed by Kadison and Singer: “They asked ‘whether or not each pure state of is the extension of some pure state of some maximal abelian algebra’ (where is the collection of bounded linear transformations on a Hilbert space.”) I heard about this question in a different formulation known as the “Bourgain-Tzafriri conjecture” (I will state it below) and the paper addresses a related well known discrepancy formulation by Weaver. (See also Weaver’s comment on the appropriate “quantum” formulation of the conjecture.)

**Updates:** A very nice post on the relation of the Kadison-Singer Conjecture to foundation of quantum mechanics is in this post in Bryan Roberts‘ blog Soul Physics. Here is a very nice post on the mathematics of the conjecture with ten interesting comments on the proof by Orr Shalit, and another nice post on Yemon Choi’s blog and how could I miss the very nice post on James Lee’s blog.. **Nov 4, 2013: **A new post with essentially the whole proof appeared on Terry tao’s blog, Real stable polynomials and the Kadison Singer Problem.

Update: A very nice blog post on the new result was written by Nikhil Srivastava on “Windows on theory.” It emphasizes the discrapancy-theoretic nature of the new result, and explains the application for partitioning graphs into expanders.

## The Bourgain-Tzafriri theorem and conjecture

Let me tell again (see this post about Lior, Michael, and Aryeh where I told it first) about a theorem of Bourgain and Tzafriri related to the Kadison-Singer conjecture.

Jean Bourgain and Lior Tzafriri considered the following scenario: Let be a real number. Let be a matrix with norm 1 and with zeroes on the diagonal. An by principal minor is “good” if the norm of is less than .

Consider the following hypergraph :

The vertices correspond to indices . A set belongs to if the sub-matrix of is good.

Bourgain and Tzafriri showed that for every there is so that for every matrix we can find so that .

Moreover, they showed that for every nonnegative weights there is so that the sum of the weights in is at least times the total weight. In other words, (by LP duality,) the vertices of the hypergraph can be fractionally covered by edges.

The “big question” is if there a real number so that for every matrix can be covered by good sets. Or, in other words, if the vertices of can be covered by edges. This question is known to be equivalent to an old conjecture by Kadison and Singer (it is also known as the “paving conjecture”). In view of what was already proved by Bourgain and Tzafriri what is needed is to show that the covering number is bounded from above by a function of the fractional covering number. So if you wish, the Kadison-Singer conjecture had become a statement about bounded integrality gap. Before proving the full result, Marcus, Spielman and Srivastava gave a new proof of the Bourgain-Tzafriti theorem.

## Additional references:

KADISON-SINGER MEETS BOURGAIN-TZAFRIRI by PETER G. CASAZZA, ROMAN VERSHYNIN, The Kadison-Singer Problem in Mathematics and Engineering: A Detailed Account pdf, and many other recent publications by Pete Casazza.

I don’t understand the statement of Conjecture 1.1 of the MSS paper you have linked. Is it not true that \sum_{i=1}^{m} | |^2 = |k|^2 \eta ???

Aargh latex! Try again . i.e. how could equation (1) hold for all vectors .

It doesn’t have to hold for all vectors u. Just those of norm 1. For example,

you could achieve this if the vectors w_i are elementary unit vectors (a 1 in one

coordinate, 0 in all the others).

Thanks! I need reading glasses…

Only example I can think of is trivial: and .

If is the union of two orthonormal bases, and , then (1) holds, and (2) works with .

BTW there is a tiny typo in Conjecture 1.1 of the paper (maybe that confused you), m should be n.

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Great achievement and very clear exposition! Roughly, the main approach, which will certainly produce many more cool results and PROOFS, can be summarized as follows: associate with

a stability preserving operation OPER, possibly from some restricted class, a functional F(p) on polynomials such that there is

a bound F(OPER(p)) \geq Const(…)F(p) and iterate this bound over the products of those preservers.

The main problem to overcome: this functional F should be close(or better equal)

on “small” degree polynomials to the thing you are really interested in. If all this done then

an easy induction proof results. When I was dealing with the Van Der Waerden- Schrijver

like inequalities, the preservers are partial differentiations with zeroing and the best functional happened to be what I called the capacity. Actually,

the capacity is essentially a measure of stability. But I understood this fact after coming up with the

idea. Interestingly, the capacity was suggested by the (numerical) approach in STOC-2000 paper

with Alex Samorodnitsky(itself motivated by two Technion’s papers on the SCALING: Nemirovski-Rothblum and David London). The functional in MSS paper is what they call “Barrier Function”, in a way a “numerical’

thing as well. I envision many more algorithmic applications of the approach as well. An interesting

extension, barely touched, is to go beyond stable polynomials. For instance, the Mixed Discriminant

is related to a nice stable polynomial(the determinant) but the Mixed Volume is not. I did some

work in this direction in http://arxiv.org/abs/0812.3687(with CONJECTURES!) and http://arxiv.org/abs/cs/0702013.

Gil, your blog and https://www.facebook.com/ellina.gurvits?fref=ts are my main two WEB favorites.

Toda Roba!

Dear Leonid, many thanks –Gil

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It is worth point out that, not only did they solve the Weaver’s conjecture, but they solve it with optimal parameters (up to constants).

Of course, the fact that they have two sets in Corollary 1.3 (i.e. r=2) is optimal, and the bound of 1/2 + O(sqrt(alpha)) that they achieve in equation (5) is also optimal, as is shown in Section 3 of Weaver’s paper.

Gil, the statement of the original form of Kadison-Singer is a bit garbled. It should be: every pure state on an atomic maximal abelian subalgebra of B(H) has a unique extension to a pure state on B(H).

Also, +1 to Anonymous for the comment about optimal parameters. Indeed, this is the best we could have hoped for!

Thanks Nik! (I am just quoting MSS quoting KS.)

Yeah, the question they quote was something asked later by Anderson and answered negatively, assuming the continuum hypothesis, by Akemann and me. See http://www.pnas.org/content/105/14/5313.full

Actually my other comment isn’t quite accurate. The question they quote was in fact asked by Kadison and Singer, but it is not what we now know as the Kadison-Singer problem. (This other question was later sharpened by Anderson, and Akemann’s and my result falsifies both versions of the question.)

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Tip my hat, again, to Adam,Dan and Nikhil! Let me stress why their proof looks so magically effortless and why some many experts failed before: the proof is not (random) matrix theoretic: matrices disappeared after the first application of the elementary first order differential operator,

but the proved Lax Conjecture(a hermitian solution due to Boris Dubrovin(1983) is sufficient)

allows to use (simple) matrix stuff to prove the needed convexity and monotonicity.

A similar observation applies to the comparison of A. Schrijver’s and mine proofs(no Lax though!,

but it was used in the earlier attempt http://arxiv.org/abs/math/0404474 and in STOC-2006 version).

On the other hand, in the context of the permanent, Lex proved much more in his paper

http://homepages.cwi.nl/~lex/files/countpms2.pdf and it has been heavily used in

http://arxiv.org/abs/1106.2844 to nail finally down Friedland’s conjecture on the monomer-dimer entropy(about the coefficients of the matching polynomial!).

I was thinking about possible extensions and generalizations. Of course, one natural question,

related to the description of maximizers is MSS paper, is to incorporate the ranks of A_i to

the (main) bound of Theorem 5.1. It looks like this body of work, starting with sparsifiers,

can be applied to an approximation of homogeneous real-stable polynomials, at least

multilinear ones, by polynomials with “few” variables. I noticed that the monotonicity/convexity

(Lemma 5.7) really needs just log-concavity, at least in one variable: take any log-concave function

F(t) that is

positive on [a, \infty) together with derivatives up to the third order. Then the ratio (F^{(1)}(t)/F(t))

is decreasing and convex on [a, \infty). With that in mind, I wonder about possible generalization

of the main (balancing) result of MSS to the Minkowski sums (aka zonotopes) of vector intervals \{a X_i: |a| \leq 1, X_i \in R^n\}. Of course, one will need to deal with mixed volumes instead of mixed discriminants,

the luxury of real roots and interlacing is gone, there are no known proved analogues of the Lax

conjectures…yet the Van der Waerden like bound was proved by an induction based

on the multiple partial differentiations.

Dear Leonid, this is very interesting. What is the generalization of MSS’s result to zonotopes you have in mind?

Dear Gil, roughly something of the kind:

Consider m symmetric vector intervals and the Minkowski(volume) polynomial

. Let be the maximum coordinate of the gradient of

the logarithm of the volume polynomial

at (1,…,1). Then there exists a partition (1,2,…,m) = such that

and .

Needless to say, it is a very preliminary statement. Can be easily rephrased in terms of

sums of support functions.

Dear Leonid, That’s very interesting! On another matter you might be interested in the recent added comments at the end of my old post on BosonSamplin. It looks that Gaussian permanents, and perhaps even general outcomes of Boson machines are very noise-sensitive in the sense of Benjamini, Schramm and me. It would be nice to prove a general theorem of this kind (for permanents it is easy), and relate it to classical simulability perhaps via your old results on Boson machines.

Dear Gil, thanks for the pointers on noise-sensitivities. I am not very familiar with this line of research. Yet, it the context of bosons I made this observation: quantum optical distributions

are much worse concentrated than their classical approximations. To be more precise:

consider a square complex $ n \times n$ unitary matrix $U$ and associate with it two homogeneous

polynomials with non-negative coefficients:

(quantum) Q(x_1,…,x_n) = Per(UDiag(x_1,…,x_n)U^{*});

(classical) C(x_1,…,x_n) = \prod_{1 \leq i \leq n} ( \sum_{1 \leq j \leq n} |U(i,j|^2 x_j).

The sum of coefficients of both of them is 1. So they both give some prob. distributions

on integer vectors \{(r_1,…,r_n): r_1+…+r_n = n \}. Moreover, both random vectors have the same

mean, namely $(1,1,…,1)$.

Now, the coefficients of Q satisfy the

upper bound $q_{r_1,…,r_n} \leq \prod_{1 \leq i \leq n} \frac{r_i !}{r_i^{r_i}};

and the coefficients of C satisfy much smaller upper bound

$c_{r_1,…,r_n} \leq \prod_{1 \leq i \leq n} \frac{1}{r_i^{r_i}}, which is due to the simple inequality

$C(x_1,…,x_n) \leq n^{-n}(x_1+….+x_n)^n$. I do conjecture that in general

$q_{r_1,…,r_n} \leq (\prod_{1 \leq i \leq n} r_i ! ) c_{r_1,…,r_n}$.

Both bounds are sharp. Also, the distribution of $C$ can be exactly efficiently classically simulated,

even though the individual probabilities are the permanents.

Actually, the polynomial $C$ is log-concave(even stable, like those in MSS paper) on the positive orthant and Q is not.

Best, Leonid.

Dear Gil, it occurred to me that there is a connection between boson sampling and Kadison-Singer Problem:

Consider linear quantum optical distribution for $n$ bosons and $m$ detectors given by $n \times m$ “unitary” matrix $U$, i.e. $UU^{*} = I$. Let $v$ be the maximum $l_2$ norm of columns of $U$.

Let $Pr(S)$ be the probability that all bosons observed by the detectors from the subset $S$.

Then there exists a partition of detectors into two sets $[1,.,.,m] = S \cup T$ such that

$(.5)^n(1 – const v)^{n} \leq Pr(S) \leq (.5)^n(1 + const v)^{n}$ and

$(.5)^n(1 – const v)^{n} \leq Pr(T) \leq (.5)^n(1 + const v)^{n}$, where $const$ is some universal constant.

Perhaps there is a direct way to prove this, i.e. without using MSS paper.

Best, Leonid.

Dear Leonid

Many thanks for the comments. this is very interesting!

Dan Spielman is gifted!

http://www.siam.org/prizes/sponsored/polya.php

Thanks for the link, and congratulations to Adam, Dan and Nikhil!

Here is a picture with Nikhil from the Simons Institute