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R E S E A R C H

Open Access

Boundedness of localization operators on

Lorentz mixed-normed modulation spaces

Ay¸se Sandıkçı

*

Dedicated to Professor Ravi P Agarwal

*Correspondence:

[email protected] Department of Mathematics, Faculty of Arts and Sciences, Ondokuz Mayıs University, Samsun, Turkey

Abstract

In this work we study certain boundedness properties for localization operators on Lorentz mixed-normed modulation spaces, when the operator symbols belong to appropriate modulation spaces, Wiener amalgam spaces, and Lorentz spaces with mixed norms.

Keywords: localization operator; Lorentz spaces; Lorentz mixed normed spaces; Lorentz mixed-normed modulation spaces; Wiener amalgam spaces

1 Introduction

In this paper we will work onRd with Lebesgue measuredx. We denote byS(Rd) the

space of complex-valued continuous functions onRdrapidly decreasing at infinity. For

any functionf :RdC, the translation and modulation operator are defined asT xf(t) =

f(t–x) andMwf(t) =eπiwtf(t) forx,w∈Rd, respectively. For ≤p≤ ∞, we write the

Lebesgue spaces (Lp(Rd), · p).

Letx,t=di=xitibe the usual scalar product onRd. The Fourier transformfˆ(orFf)

offL(Rd) is defined to be

ˆ f(t) =

Rd

f(x)e–πix,tdx.

For a fixed nonzerogS(Rd) the short-time Fourier transform (STFT) of a function fS(Rd) with respect to the windowgis defined as

Vgf(x,w) =f,MwTxg=

Rdf(t)g(t–x)e

–πitwdt,

forx,w∈Rd. Then the localization operatorAϕ,ϕ

a with symbolaand windowsϕ,ϕis

defined to be

,ϕ

a f(t) =

Rd

a(x,w)Vϕf(x,w)MwTxϕdx dw.

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IfaS(Rd) andϕ

,ϕ∈S(Rd), then the localization operator is a well-defined

continu-ous operator fromS(Rd) toS(Rd). Moreover, it is to be interpreted in a weak sense as

,ϕ

a f,g

=aVϕf,g=a,Vϕf Vϕg

forf,gS(Rd), [, ].

Fix a nonzero windowgS(Rd) and p,q≤ ∞. Then the modulation spaceMp,q(Rd)

consists of all tempered distributionsfS(Rd) such that the short-time Fourier

trans-form Vgf is in the mixed-norm space Lp,q(Rd). The norm on Mp,q(Rd) is fMp,q =

VgfLp,q. Ifp=q, then we writeMp(Rd) instead ofMp,p(Rd). Modulation spaces are

Ba-nach spaces whose definitions are independent of the choice of the windowg(see [, ]).

L(p,q) spaces are function spaces that are closely related toLp spaces. We consider

complex-valued measurable functionsf defined on a measure space (X,μ). The measure

μis assumed to be nonnegative. We assume that the functionsf are finite valued a.e. and somey> ,μ(Ey) <∞, whereEy=Ey[f] ={xX| |f(x)|>y}. Then, fory> ,

λf(y) =μ(Ey) =μ

xX|f(x)>y

is the distribution function off. The rearrangement off is given by

f∗(t) =infy> |λf(y)≤t =sup

y> |λf(y) >t

fort> . The average function off is also defined by

f∗∗(x) =  x

x

f∗(t)dt.

Note thatλf,f∗, andf∗∗are nonincreasing and right continuous functions on (,∞). If λf(y) is continuous and strictly decreasing thenf∗(t) is the inverse function ofλf(y). The

most important property off∗is that it has the same distribution function asf. It follows that

X

f(x)pdμ(x)

p =

f∗(t)pdt

p

. (.)

The Lorentz space denoted byL(p,q)(X,μ) (shortlyL(p,q)) is defined to be vector space of all (equivalence classes) of measurable functionsf such thatfpq<∞, where

fpq=

(qptqp–[f(t)]qdt)q,  <p,q<, supt>tpf(t),  <pq=.

By (.), it follows thatfpp=fp and soL(p,p) =Lp. Also,L(p,q)(X,μ) is a normed

space with the norm

fpq=

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For any one of the casesp=q= ;p=q=∞or  <p<∞and ≤q≤ ∞, the Lorentz spaceL(p,q)(X,μ) is a Banach space with respect to the norm · pq. It is also well known

that if  <p<∞, ≤q≤ ∞we have

·∗

pq≤ ·pq

p

p– ·

pq

(see [, ]).

LetXandY be two measure spaces withσ-finite measuresμandν, respectively, and

letf be a complex-valued measurable function on (X×Y,μ×ν),  <P= (p,p) <∞, and

≤Q= (q,q)≤ ∞. The Lorentz mixed norm spaceL(P,Q) =L(P,Q)(X×Y) is defined

by

L(P,Q) =L(p,q)

L(p,q)

=f :fPQ=fL(p,q)(L(p,q))=fpqpq<∞ . Thus,L(P,Q) occurs by taking anL(p,q)-norm with respect to the first variable and an

L(p,q)-norm with respect to the second variable. TheL(P,Q) space is a Banach space

under the norm · PQ(see [, ]).

Fix a window functiongS(Rd)\{}, P= (p

,p) <∞, and ≤Q= (q,q)≤ ∞.

We letM(P,Q)(Rd) denote the subspace of tempered distributionsS(Rd) consisting of

fS(Rd) such that the Gabor transformV

gf off is in the Lorentz mixed norm space

L(P,Q)(Rd). We endow it with the normfM(P,Q)=VgfPQ, where · PQis the norm

of the Lorentz mixed norm space. It is well known thatM(P,Q)(Rd) is a Banach space

and different windows yield equivalent norms. If p=q =pandp=q=q, then the

spaceM(P,Q)(Rd) is the standard modulation spaceMp,q(Rd), and ifP=pandQ=q, in

this caseM(P,Q)(Rd) =M(p,q)(Rd) (see [, ]), where the spaceM(p,q)(Rd) is Lorentz

type modulation space (see []). Furthermore, the spaceM(p,q)(Rd) was generalized to

M(p,q,w)(Rd) by taking weighted Lorentz space rather than Lorentz space (see [, ]). In this paper, we will denote the Lorentz space byL(p,q), the Lorentz mixed norm space

byL(P,Q), the standard modulation space byMp,q, the Lorentz type modulation space by

M(p,q), and the Lorentz mixed-normed modulation space byM(P,Q).

Let ≤r,s≤ ∞. Fix a compactQ⊂Rdwith nonempty interior. Then the Wiener

amal-gam space W(Lr,Ls)(Rd) with local componentLr(Rd) and global componentLs(Rd) is

defined as the space of all measurable functionsf :Rd→Csuch thatfχKLr(Rd) for

each compact subsetK⊂Rd, for which the norm

fW(Lr,Ls)=Ffs=Q+xr

s

is finite, whereχKis the characteristic function ofKand

Ff(x) =fχQ+xrLs

Rd.

It is known that ifr≥rands≤sthenW(Lr,Ls)(Rd)⊂W(Lr,Ls)(Rd). Ifr=sthen

W(Lr,Lr)(Rd) =Lr(Rd) (see [–]).

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norms. Our results extend some results in [, ] to the Lorentz mixed-normed modula-tion spaces.

2 Boundedness of localization operators on Lorentz mixed normed modulation spaces

We start with the following lemma, which will be used later on.

Lemma . Let

P+

P = ,

Q+

Q ≥,fL(P,Q)(R

d),hL(P,Q

)(Rd).Then fh

L∞(Rd)and

L(P,Q)

RdLP,Q

RdLRd (.)

with the norm inequality

fh∞≤ fPQhPQ, (.)

where P= (p,p),Q= (Q,Q),Q= (Q,Q).

Proof It is well known that there areL(p,q)∗L(p,q)L∞convolution relations

be-tween Lorentz spaces and

fh∞≤ fpqhpq,

where 

p +

p = ,

q +

q ≥, by Theorem . in []. Then (.) and (.) can easily be

verified by using iteration and the one variable proofs given in [].

LetgD(Rd) be a test function such that

x∈ZdTxg≡. LetX(Rd) be a translation

invariant Banach space of functions with the property thatD·XX. In the spirit of [,

], the Wiener amalgam spaceW(X,L(P,Q)) with local componentXand global

compo-nentL(P,Q) is defined as the space of all functions or distributions for which the norm

fW(X,L(P,Q))=f·T(z,z)gXPQ

is finite, where ≤P<∞, ≤Q≤ ∞. Moreover, different choices ofgDyield equiva-lent norms and give the same space.

The boundedness of,ϕ

Mζa foraM

is established by our next theorem. The proof is

similar to Lemma . in [] but let us provide the details anyway, for completeness’ sake.

Theorem .

(i) Let <P<∞,≤Q<∞.IffM(P,Q)(Rd)andgM(Rd),then

VgfW(FL,L(P,Q))(Rd)with

VgfW(FL,L(P,Q))fM(P,Q)gM.

(ii) Let

P+

P = ,

Q +

Q ≥.IffM(P,Q)(R

d)andgM(P,Q

)(Rd),then

VgfW(FL,L∞)(Rd)with

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Proof (i) LetϕS(Rd)\{}and set=VϕϕS(Rd). By using the equalityV

gf(x,w) =

(f ·Txg)∧(w), we write

Vgf ·T(z,z)FL=

Rd

(Vgf ·T(z,z))

(t)dt

=

Rd

VVgf(z,z,t,t)dtdt

=

Rd

Vϕg(–z–t,t)Vϕf(–t,z+t)dtdt

=

Rd

Vϕf(u,u)Vϕg(u–z,u–z)dudu

=|Vϕf| ∗ |Vϕg|∼(z,z), (.)

for f,gS(Rd), where (Vϕg)∼(z) = (Vϕg)(–z), z ∈Rd. Since f,gS(Rd), then f

M(P,Q)(Rd) andgM(Rd) by Proposition  in []. SoVϕfL(P,Q)(Rd) andVϕg

L(Rd). Then, by Proposition  in [], we obtain

VgfW(FL,L(P,Q))=Vgf ·T(z,z)FLPQ =|Vϕf| ∗ |Vϕg|∼PQ

VϕfPQVϕg

=fM(P,Q)gM. (.)

This completes the proof.

(ii) Using Lemma . and (.), we have

VgfW(FL,L)=|Vϕf| ∗ |Vϕg|∼VϕfPQVϕgPQ=fM(P,Q)gM(P,Q).

Theorem . Let <P<∞, ≤Q<∞.If aM∞(Rd),ϕ

,ϕ∈M(Rd),then AϕMζ,ϕais

bounded on M(P,Q)(Rd)for everyζRdwith

Aϕ,ϕ

MζaB(M(P,Q))≤ aMϕMϕM.

Proof LetfM(P,Q)(Rd) andgM(P,Q)(Rd), where

P+

P = ,

Q+

Q = . Then we

writefW(FL,L(P,Q))(Rd) andgW(FL,L(P,Q))(Rd) by above theorem. Moreover, sinceM(, )(Rd) =M(Rd), we haveW(FL,L) =M=M(, ) by []. Hence

using the Hölder inequalities for Wiener amalgam spaces [] and (.) we obtain

f ·gM =f·gW(FL,L)

fW(FL,L(P,Q))gW(FL,L(P,Q))

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Thus by using (.) we have

,ϕ

Mζaf,g=Mζa,Vϕf·gMζaM(∞,∞)f·gM(,) ≤ aMϕMϕMfM(P,Q)gM(P,Q).

Hence we get

,ϕ

MζaB(M(P,Q))≤ aMϕMϕM.

Theorem . LetϕS(Rd)\{}be a window function.If <P,Q<,t (,),st

r and aW(Lr,Ls),then

M,ϕ

ζa:M(tP,tQ)

RdMtP,tQRd

is bounded for everyζ ∈Rd,where

P+

P = ,

Q+

Q = ,and

t +

t = ,and the operator

norm satisfies the estimate

M,ϕ

ζaaW(Lr,Ls).

Proof Let t < ∞, fM(tP,tQ)(Rd), and h M(tP,tQ)(Rd). Then we have V

ϕf

L(tP,tQ)(Rd) andV

ϕhL(tP,tQ)(Rd). SinceVϕfL(tP,tQ)(Rd), thenVϕf(tP)(tQ)<

∞. By using the equality (.) in [], we get

Vϕf(tP)(tQ)=Vϕf(tp)(tq)(tp

)(tq)=|Vϕf|

tpq

t

(tp)(tq) =|Vϕf|t

pq

tt

pq 

t =|V

ϕf|tpq

pq

t

=|Vϕf|tPQ

t. (.)

Hence we have|Vϕf|tL(P,Q)(Rd). Similarly,|Vϕh|tL(P,Q)(Rd). By the Hölder

in-equality for Lorentz spaces with mixed norm and (.) we have

Vϕf ·Vϕhtt=|Vϕf|t|Vϕh|t≤|Vϕf|tPQ|Vϕh|tPQ

=Vϕft(tP)(tQ)Vϕht(tP)(tQ). (.)

Since aW(Lr,Ls), then M

ζaW(Lr,Ls) for everyζ ∈Rd. Also since W(Lr,Ls)⊂

W(Lt,Lt) =Lt(Rd), then we have

at =MζatMζaW(Lr,Ls)=aW(Lr,Ls). (.)

By using (.), (.), and applying again the Hölder inequality, we get

M,ϕζaf,h=MζaVϕf,Vϕh

Rd

Mζa(x,w)(Vϕf·Vϕh)(x,w)dx dw

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atVϕf(tP)(tQ)Vϕh(tP)(tQ)

aW(Lr,Ls)fM(tP,tQ)hM(tP,tQ). (.)

If (tp), (tq) =∞, then (M((tP), (tQ))(Rd))=M(tP,tQ)(Rd) by Theorem  in []. Thus

we have from (.) that

M,ϕ

ζafM((tP),(tQ))= sup =h∈M(tP,tQ)

|M,ϕ ζaf,h| hM(tP,tQ) ≤

aW(Lr,Ls)fM(tP,tQ).

HenceM,ϕ

ζais bounded. Also we have

M,ϕ

ζa= sup

=f∈M(tP,tQ)

M,ϕζafM((tP),(tQ))

fM(tP,tQ) ≤

aW(Lr,Ls).

Theorem . Letϕ≤R,S<M(R,S)(Rd),where R= (r

,r),S= (s,s).If≤sr≤ ∞

and aW(Lr,Ls)then

M,ϕζa:M(P,Q)RdM(P,Q)Rd

is bounded for everyζ∈Rd,with

M,ϕ

ζaCaW(Lr,Ls)

for some C> .

Proof SinceaW(Lr,Ls), thenM

ζaW(Lr,Ls) for everyζ ∈Rd. Also sincesr, there

exists ≤t≤ ∞such thatst≤r. ThenW(Lr,Ls)(Rd)⊂Lt(Rd) and

Mζat=at≤ aW(Lr,Ls)=MζaW(Lr,Ls) (.)

for allaW(Lr,Ls)(Rd). LetB(M(P,Q)(Rd),M(P,Q)(Rd)) be the space of the bounded

linear operators from M(P,Q)(Rd) into M(P,Q)(Rd). Also let T be an operator from

L(Rd) intoB(M(P,Q)(Rd),M(P,Q)(Rd)) byT(a) =Aϕ,ϕ

Mζa. Take anyfM(P,Q)(R

d) and

hM(P,Q)(Rd). Assume thataW(L,L)(Rd) =L(Rd). By the Hölder inequality we get

T(a)f,h=M,ϕζaf,h=MζaVϕf,Vϕh

Rd

Mζa(x,w)Vϕf(x,w)Vϕh(x,w)dx dw

=

Rd

a(x,w)f,MwTxϕh,MwTxϕdx dw

Rd

a(x,w)fM(P,Q)MwTxϕM(P,Q)hM(P,Q)

× MwTxϕM(P,Q)dx dw

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Hence by (.)

T(a)fM(P,Q)=AMϕ,ϕζafM(P,Q)= sup

=h∈M(P,Q)

|M,ϕζaf,h| hM(P,Q)

ϕM(P,Q)ϕM(P,Q)fM(P,Q)a.

Then

T(a)=M,ϕ

ζa= sup

=f∈M(P,Q)

M,ϕ

ζafM(P,Q) fM(P,Q) ≤

ϕM(P,Q)ϕM(P,Q)a. (.)

Thus the operator

T:LRdBM(P,Q)Rd,M(P,Q)Rd (.)

is bounded. Now let aW(L∞,L∞)(Rd) =L∞(Rd). Take any fM(P,Q)(Rd) and hM(P,Q)(Rd). ThenVϕfL(P,Q)(Rd),VϕhL(P,Q)(Rd). Applying the Hölder

in-equality

T(a)f,h=M,ϕζaf,h=MζaVϕf,Vϕh

Rd

Mζa(x,w)Vϕf(x,w)Vϕh(x,w)dx dw

a

Rd

Vϕf(x,w)Vϕh(x,w)dx dw

aVϕfPQVϕhPQ. (.)

By using (.) we write

T(a)fM(P,Q)=AMϕ,ϕζafM(P,Q)= sup

=h∈M(P,Q)

|M,ϕζaf,h| hM(P,Q) ≤

afM(P,Q). (.)

Hence by (.)

T(a)=M,ϕζa= sup

=f∈M(P,Q)

M,ϕζafM(P,Q)

fM(P,Q) ≤

a∞.

That means the operator

T:L∞RdBM(P,Q)Rd,M(P,Q)Rd (.)

is bounded. Combining (.) and (.) we obtain

T:LtRdBM(P,Q)Rd,M(P,Q)Rd

is bounded by interpolation theorem for ≤t≤ ∞. That means the localization operator

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is bounded for ≤t≤ ∞. Hence there existsC>  such that

T(a)=M,ϕζaCat. (.)

This implies that it is also true for ≤t≤ ∞. From (.) and (.) we write

T(a)=M,ϕ

ζaCat≤CaW(Lr,Ls).

Proposition . Letϕ≤R,S<M(R,S)(Rd),where R= (r

,r),S= (s,s).If <s≤

and aW(L,Ls)(Rd)then

M,ϕζa:M(P,Q)RdM(P,Q)Rd

is bounded.

Proof Let  <s≤ and let aW(L,Ls)(Rd). Then M

ζaW(L,Ls) for every ζ

Rd. Since W(L,Ls)(Rd)L(Rd), there exists a numberC>  such that M ζa ≤

CMζaW(L,Ls). Hence by (.),

M,ϕζaϕM(P,Q)ϕM(P,Q)Mζa

CϕM(P,Q)ϕM(P,Q)MζaW(L,Ls) =CϕM(P,Q)ϕM(P,Q)aW(L,Ls).

Then the localization operator from M(P,Q)(Rd) into M(P,Q)(Rd) is bounded for

 <s≤.

Proposition . Letϕ≤R,S<M(R,S)(Rd),where R= (r,r),S= (s,s).If≤P,Q<

and aL(P,Q)(Rd)then the localization operator

M,ϕ

ζa:M(P,Q)

RdM(P,Q)Rd

is bounded,where

P+

P = ,

Q+

Q = .

Proof Let aL(P,Q)(Rd). Then M

ζaL(P,Q)(Rd) for every ζ ∈ Rd with

MζaPQ =aPQ. Take anyfM(P,Q)(Rd) andhM(P,Q)(Rd). Applying the Hölder

inequality we have by (.)

M,ϕζaf,h

Rd

Mζa(x,w)Vϕf(x,w)h,MwTxϕdx dw

Rd

a(x,w)Vϕf(x,w)hM(P,Q)MwTxϕM(P,Q)dx dw

=hM(P,Q)ϕM(P,Q)

Rd

a(x,w)Vϕf(x,w)dx dw

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Similarly to (.), we get

M,ϕ

ζaϕM(P,Q)aPQ.

Then the localization operatorM,ϕζafromM(P,Q)(Rd) intoM(P,Q)(Rd) is bounded.

Corollary . It is known by Propositionin[]thatS(Rd)M(R,S)(Rd)forR,S<

∞.ThenS(Rd)

≤R,S<∞M(R,S)(Rd).So,Theorem.,Propositions.and.are still

true under the same hypotheses for them ifϕS(Rd).

Corollary . It is known[]that if P=p and Q=q,then Lorentz mixed-normed

modu-lation space M(P,Q)(Rd)is the Lorentz type modulation space M(p,q)(Rd).Therefore our

theorems hold for a Lorentz type modulation space rather than for a Lorentz mixed-normed modulation space.

Competing interests

The author declares that she has no competing interests.

Received: 21 August 2014 Accepted: 24 October 2014 Published:31 Oct 2014

References

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10.1186/1029-242X-2014-430

Cite this article as:Sandıkçı:Boundedness of localization operators on Lorentz mixed-normed modulation spaces.

References

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