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For certain applications in linear algebra, it is useful to know properties of the probability distribution of the largest eigenvalue of a finite sum of random matrices. Suppose {Xk} is a finite sequence of random matrices. Analogous to the well-known Chernoff bound for sums of scalars, a bound on the following is sought for a given parameter t:

Pr{λmax(kXk)t}

The following theorems answer this general question under various assumptions; these assumptions are named below by analogy to their classical, scalar counterparts. All of these theorems can be found in Template:Harv, as the specific application of a general result which is derived below. A summary of related works is given.

Matrix Gaussian and Rademacher series

Self-adjoint matrices case

Consider a finite sequence {Ak} of fixed, self-adjoint matrices with dimension d, and let {ξk} be a finite sequence of independent standard normal or independent Rademacher random variables.

Then, for all t0,

Pr{λmax(kξkAk)t}det2/2σ2

where

σ2=kAk2.

Rectangular case

Consider a finite sequence {Bk} of fixed, self-adjoint matrices with dimension d1×d2, and let {ξk} be a finite sequence of independent standard normal or independent Rademacher random variables. Define the variance parameter

σ2=max{kBkBk*,kBk*Bk}.

Then, for all t0,

Pr{kξkBkt}(d1+d2)et2/2σ2.

Matrix Chernoff inequalities

The classical Chernoff bounds concerns the sum of independent, nonnegative, and uniformly bounded random variables. In the matrix setting, the analogous theroem concerns a sum of positive-semidefinite random matrices subjected to a uniform eigenvalue bound.

Matrix Chernoff I

Consider a finite sequence {Xk} of independent, random, self-adjoint matrices with dimension d. Assume that each random matrix satisfies

Xk0andλmax(Xk)R

almost surely.

Define

μmin=λmin(k𝔼Xk)andμmax=λmax(k𝔼Xk).

Then

Pr{λmin(kXk)(1δ)μmin}d[eδ(1δ)1δ]μmin/Rfor δ[0,1], and
Pr{λmax(kXk)(1+δ)μmax}d[eδ(1+δ)1+δ]μmax/Rfor δ0.

Matrix Chernoff II

Consider a sequence {Xk:k=1,2,,n} of independent, random, self-adjoint matrices that satisfy

Xk0andλmax(Xk)1

almost surely.

Compute the minimum and maximum eigenvalues of the average expectation,

μ¯min=λmin(1nk=1n𝔼Xk)andμ¯max=λmax(1nk=1n𝔼Xk).

Then

Pr{λmin(1nk=1nXk)α}denD(αμ¯min)for 0αμ¯min, and
Pr{λmax(1nk=1nXk)α}denD(αμ¯max)for μ¯maxα1.

The binary information divergence is defined as

D(au)=a(logalogu)+(1a)(log(1a)log(1u))

for a,u[0,1].

Matrix Bennett and Bernstein inequalities

In the scalar setting, Bennett and Bernstein inequalities describe the upper tail of a sum of independent, zero-mean random variables that are either bounded or subexponential. In the matrix case, the analogous results concern a sum of zero-mean random matrices.

Bounded case

Consider a finite sequence {Xk} of independent, random, self-adjoint matrices with dimension d. Assume that each random matrix satisfies

Xk0andλmax(Xk)R

almost surely.

Compute the norm of the total variance,

σ2=k𝔼(Xk2).

Then, the following chain of inequalities holds for all t0:

Pr{λmax(kXk)t}dexp(σ2R2h(Rtσ2))dexp(t2σ2+Rt/3){dexp(3t2/8σ2)for tσ2/R;dexp(3t/8R)for tσ2/R.

The function h(u) is defined as h(u)=(1+u)log(1+u)u for u0.

Subexponential case

Consider a finite sequence {Xk} of independent, random, self-adjoint matrices with dimension d. Assume that

𝔼Xk=0and𝔼(Xkp)p!2Rp2Ak2

for p=2,3,4,.

Compute the variance parameter,

σ2=kAk2.

Then, the following chain of inequalities holds for all t0:

Pr{λmax(kXk)t}dexp(t2/2σ2+Rt){dexp(t2/4σ2)for tσ2/R;dexp(t/4R)for tσ2/R.

Rectangular case

Consider a finite sequence {Zk} of independent, random, matrices with dimension d1×d2. Assume that each random matrix satisfies

𝔼Zk=0andZkR

almost surely. Define the variance parameter

σ2=max{k𝔼(ZkZk*),k𝔼(Zk*Zk)}.

Then, for all t0

Pr{kZkt}(d1+d2)exp(t2σ2+Rt/3)

Matrix Azuma, Hoeffding, and McDiarmid inequalities

Matrix Azuma

The scalar version of Azuma's inequality states that a scalar martingale exhibits normal concentration about its mean value, and the scale for deviations is controlled by the total maximum squared range of the difference sequence. The following is the extension in matrix setting.

Consider a finite adapted sequence {Xk} of self-adjoint matrices with dimension d, and a fixed sequence {Ak} of self-adjoint matrices that satisfy

𝔼k1Xk=0andXk2Ak2

almost surely.

Compute the variance parameter

σ2=kAk2.

Then, for all t0

Pr{λmax(kXk)t}det2/8σ2

The constant 1/8 can be improved to 1/2 when there is additional information available. One case occurs when each summand Xk is conditionally symmetric. Another example requires the assumption that Xk commutes almost surely with Ak.

Matrix Hoeffding

Placing addition assumption that the summands in Matrix Azuma are independent gives a matrix extension of Hoeffding's inequalities.

Consider a finite sequence {Xk} of independent, random, self-adjoint matrices with dimension d, and let {Ak} be a sequence of fixed self-adjoint matrices. Assume that each random matrix satisfies

𝔼Xk=0andXk2Ak2

almost surely.

Then, for all t0

Pr{λmax(kXk)t}det2/8σ2

where

σ2=kAk2.

Matrix bounded difference (McDiarmid)

In scalar setting, McDiarmid's inequality provides one common way of bounding the differences by applying Azuma's inequality to a Doob martingale. A version of the bounded differences inequality holds in the matrix setting.

Let {Zk:k=1,2,,n} be an independent, family of random variables, and let H be a function that maps n variables to a self-adjoint matrix of dimension d. Consider a sequence {Ak} of fixed self-adjoint matrices that satisfy

(H(z1,,zk,,zn)H(z1,,z'k,,zn))2Ak2,

where zi and z'i range over all possible values of Zi for each index i. Compute the variance parameter

σ2=kAk2.

Then, for all t0

Pr{λmax(H(z)𝔼H(z))t}det2/8σ2,

where z=(Z1,,Zn).

Survey of related theorems

The first bounds of this type were derived by Template:Harv. Recall the theorem above for self-adjoint matrix Gaussian and Rademacher bounds: For a finite sequence {Ak} of fixed, self-adjoint matrices with dimension d and for {ξk} a finite sequence of independent standard normal or independent Rademacher random variables, then

Pr{λmax(kξkAk)t}det2/2σ2

where

σ2=kAk2.

Ahlswede and Winter would give the same result, except with

σAW2=kλmax(Ak2).

By comparison, the σ2 in the theorem above commutes Σ and λmax; that is, it is the largest eigenvalue of the sum rather than the sum of the largest eigenvalues. It is never larger than the Ahlswede–Winter value (by the norm triangle inequality), but can be much smaller. Therefore, the theorem above gives a tighter bound than the Ahlswede–Winter result.

The chief contribution of Template:Harv was the extension of the Laplace-transform method used to prove the scalar Chernoff bound (see Chernoff bound#Theorem for additive form (absolute error)) to the case of self-adjoint matrices. The procedure given in the derivation below. All of the recent works on this topic follow this same procedure, and the chief differences follow from subsequent steps. Ahlswede & Winter use the Golden–Thompson inequality to proceed, whereas Tropp Template:Harv uses Lieb's Theorem.

Suppose one wished to vary the length of the series (n) and the dimensions of the matrices (d) while keeping the right-hand side approximately constant. Then n must vary approximately as the log of d. Several papers have attempted to establish a bound without a dependence on dimensions. Rudelson and Vershynin Template:Harv give a result for matrices which are the outer product of two vectors. Template:Harv provide a result without the dimensional dependence for low rank matrices. The original result was derived independently from the Ahlswede–Winter approach, but Template:Harv proves a similar result using the Ahlswede–Winter approach.

Finally, Oliveira Template:Harv proves a result for matrix martingales independently from the Ahlswede–Winter framework. Tropp Template:Harv slightly improves on the result using the Ahlswede–Winter framework. Neither result is presented in this article.

Derivation and proof

Ahlswede and Winter

The Laplace transform argument found in Template:Harv is a significant result in its own right: Let Y be a random self-adjoint matrix. Then

Pr{λmax(Y)t}infθ>0{eθtE[treθY]}.

To prove this, fix θ>0. Then

Pr{λmax(Y)t}=Pr{λmax(θY)θt}=Pr{eλmax(θY)eθt}eθtEeλmax(θY)eθtEtre(θY)

The second-to-last inequality is Markov's inequality. The last inequality holds since eλmaxθY=λmaxeθYtreθY. Since the left-most quantity is independent of θ, the infimum over θ>0 remains an upper bound for it.

Thus, our task is to understand EtreθY Nevertheless, since trace and expectation are both linear, we can commute them, so it is sufficient to consider EeθY:=MY(θ), which we call the matrix generating function. This is where the methods of Template:Harv and Template:Harv diverge. The immediately following presentation follows Template:Harv.

The Golden–Thompson inequality implies that

trMX1+X2(θ)tr[(EeθX1)(EeθX2)]=trMX1(θ)MX2(θ), where we used the linearity of expectation several times.

Suppose Y=kXk. We can find an upper bound for trMY(θ) by iterating this result. Noting that tr(AB)tr(A)λmax(B), then

trMY(θ)tr[(Eek=1n1θXk)(EeθXn)]tr(Eek=1n1θXk)λmax(EeθXn).

Iterating this, we get

trMY(θ)(trI)[Πkλmax(EeθXk)]=dekλmax(logEeθXk)

So far we have found a bound with an infimum over θ. In turn, this can be bounded. At any rate, one can see how the Ahlswede–Winter bound arises as the sum of largest eigenvalues.

Tropp

The major contribution of Template:Harv is the application of Lieb's theorem where Template:Harv had applied the Golden–Thompson inequality. Tropp's corollary is the following: If H is a fixed self-adjoint matrix and X is a random self-adjoint matrix, then

EtreH+XtreH+log(EeX)

Proof: Let Y=eX. Then Lieb's theorem tells us that

f(Y)=treH+log(Y)

is concave. The final step is to use Jensen's inequality to move the expectation inside the function:

EtreH+log(Y)treH+log(EY).

This gives us the major result of the paper: the subadditivity of the log of the matrix generating function.

Subadditivity of log mgf

Let Xk be a finite sequence of independent, random self-adjoint matrices. Then for all θ,

trMkXk(θ)treklogMXk(θ)

Proof: It is sufficient to let θ=1. Expanding the definitions, we need to show that

EtrekθXktreklogEeθXk.

To complete the proof, we use the law of total expectation. Let Ek be the expectation conditioned on X1,,Xk. Since we assume all the Xi are independent,

Ek1eXk=EeXk.

Define Ξk=logEk1eXk=logMXk(θ).

Finally, we have

Etrek=1nXk=E0En1trek=1n1Xk+XnE0En2trek=1n1Xk+log(En1eXn)=E0En2trek=1n2Xk+Xn1+Ξn=trek=1nΞk

where at every step m we use Tropp's corollary with

Hm=k=1m1Xk+k=m+1nΞk

Master tail bound

The following is immediate from the previous result:

Pr{λmax(kXk)t}infθ>0{eθttreklogMXk(θ)}

All of the theorems given above are derived from this bound; the theorems consist in various ways to bound the infimum. These steps are significantly simpler than the proofs given.

References

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    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

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    Discover out more about real estate funding in the area, together with info on international funding incentives and property possession. Many Singaporeans have been investing in property across the causeway in recent years, attracted by comparatively low prices. However, those who need to exit their investments quickly are likely to face significant challenges when trying to sell their property – and could finally be stuck with a property they can't sell. Career improvement programmes, in-house valuation, auctions and administrative help, venture advertising and marketing, skilled talks and traisning are continuously planned for the sales associates to help them obtain better outcomes for his or her shoppers while at Knight Frank Singapore. No change Present Rules

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    A vendor's stamp duty has been launched on industrial property for the primary time, at rates ranging from 5 per cent to 15 per cent. The Authorities might be trying to reassure the market that they aren't in opposition to foreigners and PRs investing in Singapore's property market. They imposed these measures because of extenuating components available in the market." The sale of new dual-key EC models will even be restricted to multi-generational households only. The models have two separate entrances, permitting grandparents, for example, to dwell separately. The vendor's stamp obligation takes effect right this moment and applies to industrial property and plots which might be offered inside three years of the date of buy. JLL named Best Performing Property Brand for second year running

    The data offered is for normal info purposes only and isn't supposed to be personalised investment or monetary advice. Motley Fool Singapore contributor Stanley Lim would not personal shares in any corporations talked about. Singapore private home costs increased by 1.eight% within the fourth quarter of 2012, up from 0.6% within the earlier quarter. Resale prices of government-built HDB residences which are usually bought by Singaporeans, elevated by 2.5%, quarter on quarter, the quickest acquire in five quarters. And industrial property, prices are actually double the levels of three years ago. No withholding tax in the event you sell your property. All your local information regarding vital HDB policies, condominium launches, land growth, commercial property and more

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