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# The Nullspace Property

Module by: Ronald DeVore. E-mail the author

We begin with a property of the null space NNN which is at the heart of proving results on instance-optimality.

We say that NNN has the Null Space Property if for all ηNηNη in N and all TTT with #Tk#Tkitalic "#" T <= k we have η X c 1 η T C X η X c 1 η T C X

Intuitively, NSP implies that for any vector in the nullspace the energy will not be concentrated in a small number of entries.

The following are equivalent formulations for NSP XXX for kkk :

1. ηXc1σk(η)ηXc1σk(η)
2. η T X c 1 η T C X η T X c 1 η T C X where η = η T + η T C η = η T + η T C .

Note also that the triangle inequality can be used as follows

η X = η T + η T C X η T X + η T C X η X = η T + η T C X η T X + η T C X

which shows that (b) is equivalent to NSP.

## Theorem 1

1. If (Φ,Δ)(Φ,Δ) $${Φ , Δ}$$ is instance optimal on XXX for the value kkk , then ΦΦΦ satisfies the NSP for 2k2k2 k on XXX with an equivalent constant.
2. If ΦΦΦ has the NSP for XXX and 2k2k2 k then ΔΔ exists Δ s.t. ΦΦΦ has the instance optimal property for kkk .

### Proof

We will prove a slightly weaker version of this to save time. We first prove that instance optimality for kkk implies NSP XXX for kkk (hence this is slightly weaker than advertised) . Let ηNηNη in N and set z=Δ(0)z=Δ(0)z =Δ { $$0$$} then

η z c 0 σ k ( η ) η + z c 0 σ k ( η ) η max { η z , η + z } c 0 σ k ( η ) instance optimal property z N triangle inequality η z c 0 σ k ( η ) η + z c 0 σ k ( η ) η max { η z , η + z } c 0 σ k ( η ) instance optimal property z N triangle inequality
(1)

We now prove 2. Suppose ΦΦΦ has the NSP for 2k2k2 k . Given yyy , (y)={x:Φ(x)=y}(y)={x:Φ(x)=y}{ $$y$$} ={ lbrace {x : Φ { $$x$$} = y} rbrace}. Let us define the decoder ΔΔΔ by Δ ( y ) : = arg min { σ K ( x ) X : x ( y ) } Δ ( y ) : = arg min { σ K ( x ) X : x ( y ) } , then

x Δ ( Φ ( x ) ) X = x x X c 1 σ 2 K ( x x ) c 1 ( σ K ( x ) σ K ( x ) ) specific 2K term approximation 2 c 1 σ K ( x ) x Δ ( Φ ( x ) ) X = x x X c 1 σ 2 K ( x x ) c 1 ( σ K ( x ) σ K ( x ) ) specific 2K term approximation 2 c 1 σ K ( x )

QED.

Note that the instance optimal property automatically gives reproduction of KKK -sparse signals.

At this stage the challenge is to create ΦΦΦ with this instance optimal property. For this we shall use the restricted isometry property as introduced earlier and which we now recall.

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