Entropy and total variation distance

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Let $X$, $Y$ be discrete random variables taking values within the same set of $N$ elements. Let the total variation distance $|P-Q|$ (which is half the $L_1$ distance between the distributions of $P$ and $Q$) be at most $epsilon$. What is a standard, easily provable bound on the difference between the entropies of $X$ and $Y$?



(It is easy to see that such a bound must depend not only on $epsilon$ but also on $N$.)










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    Let $X$, $Y$ be discrete random variables taking values within the same set of $N$ elements. Let the total variation distance $|P-Q|$ (which is half the $L_1$ distance between the distributions of $P$ and $Q$) be at most $epsilon$. What is a standard, easily provable bound on the difference between the entropies of $X$ and $Y$?



    (It is easy to see that such a bound must depend not only on $epsilon$ but also on $N$.)










    share|cite|improve this question

























      up vote
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      Let $X$, $Y$ be discrete random variables taking values within the same set of $N$ elements. Let the total variation distance $|P-Q|$ (which is half the $L_1$ distance between the distributions of $P$ and $Q$) be at most $epsilon$. What is a standard, easily provable bound on the difference between the entropies of $X$ and $Y$?



      (It is easy to see that such a bound must depend not only on $epsilon$ but also on $N$.)










      share|cite|improve this question















      Let $X$, $Y$ be discrete random variables taking values within the same set of $N$ elements. Let the total variation distance $|P-Q|$ (which is half the $L_1$ distance between the distributions of $P$ and $Q$) be at most $epsilon$. What is a standard, easily provable bound on the difference between the entropies of $X$ and $Y$?



      (It is easy to see that such a bound must depend not only on $epsilon$ but also on $N$.)







      pr.probability probability-distributions inequalities it.information-theory entropy






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      edited 2 hours ago









      Iosif Pinelis

      14.8k12154




      14.8k12154










      asked 4 hours ago









      H A Helfgott

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          $newcommanddedelta
          newcommandepepsilon$



          Note that $pln p-p$ is decreasing in $pin[0,1]$, so that $pln p-ple qln q-q$ and hence $pln p-qln qle p-q=|p-q|$ if $0le qle ple1$.



          Next, take any real $cge1$. Note that $g(p):=pln p+cp$ (with $g(0):=0$) is convex in $pin[0,1]$. So, assuming $0le ple qle1$ and $qge e^-c$, we have
          $g(p)le g(0)vee g(q)=0vee g(q)=g(q)$ and hence
          $pln p-qln qle c(q-p)=c|p-q|$.
          Thus,
          beginequation
          pln p-qln qle c|p-q|
          endequation
          whenever $0le p,qle1$ and $qge e^-c$.



          Also, $-qln q$ is increasing in $qin[0,e^-1]$ and hence in $qin[0,e^-c]$, so that $-qln qle ce^-c$ for $qin[0,e^-c]$. Also, $pln ple0$ if $0le ple1$.



          Therefore, the difference between the entropies of $Q=(q_i)_i=1^N$ and $P=(p_i)_i=1^N$ is
          beginequation
          sum_1^N p_iln p_i-sum_1^N q_iln q_ile S_1+S_2+S_3,
          endequation
          where
          beginalign*
          S_1 &:=sum_icolon q_ige e^-c (p_iln p_i-q_iln q_i)le sum_icolon q_ige e^-cc|p_i-q_i|
          le cde quadtextif dege|P-Q|_1, \
          S_2 &:= sum_icolon q_i< e^-cp_iln p_ile0, \
          S_3 &:= sum_icolon q_i< e^-c(-q_iln q_i)lesum_icolon q_i< e^-cce^-cle Nce^-c.
          endalign*
          So, taking now $c=lnfrac Nde$ and assuming $Nge ede$, we see that
          beginequation
          sum_1^N p_iln p_i-sum_1^N q_iln q_ile
          cde+Nce^-c
          =2delnfrac Nde.
          endequation
          Taking here $de=ep/2$ and noting that $Nge1$, we conclude that
          beginequation
          sum_1^N p_iln p_i-sum_1^N q_iln q_ile
          eplnfrac2Nep
          endequation
          if $eple2/e$.






          share|cite|improve this answer





























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            Claim. If $|P-Q|leqvarepsilonleqfrac12$, then $|H(P)-H(Q)| leq H(varepsilon) + varepsilonlog N$.



            Proof.
            Let $varepsilon':=|P-Q|$.
            Let $(X,Y)$ be an optimal coupling of $P$ and $Q$, so that
            beginalign
            mathbbP(Xneq Y) = |P-Q| ;.
            endalign
            Using a standard construction, we can assume that $X$ and $Y$ have the particular form
            beginalign
            X &:=
            begincases
            Z & textif $B=0$, \
            tildeX & textif $B=1$,
            endcases &
            Y &:=
            begincases
            Z & textif $B=0$, \
            tildeY & textif $B=1$,
            endcases
            endalign
            where $B$, $Z$ and $(tildeX,tildeY)$ are independent and $BsimtextBern(varepsilon')$.



            Note that
            beginalign
            H(X|B) leq H(X) leq H(B) + H(X|B) ;.
            endalign
            For $H(X|B)$ we can write
            beginalign
            H(X|B) &= varepsilon' H(X|B=1) + (1-varepsilon') H(X|B=0) \
            &= varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;.
            endalign
            Thus,
            beginalign
            varepsilon' H(tildeX) + (1-varepsilon') H(Z) &leq H(X)
            leq H(B) + varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;,
            tag$clubsuit$
            endalign
            and similarly,
            beginalign
            varepsilon' H(tildeY) + (1-varepsilon') H(Z) &leq H(Y)
            leq H(B) + varepsilon' H(tildeY) + (1-varepsilon') H(Z) ;.
            tag$spadesuit$
            endalign



            Combining ($clubsuit$) and ($spadesuit$) we get
            beginalign
            |H(X)-H(Y)| &leq
            H(B) + varepsilon' |H(tildeX) - H(tildeY)| \
            &leq H(varepsilon') + varepsilon' log N \
            &leq H(varepsilon) + varepsilon log N ;,
            endalign
            as claimed.
            QED






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              2 Answers
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              up vote
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              down vote













              $newcommanddedelta
              newcommandepepsilon$



              Note that $pln p-p$ is decreasing in $pin[0,1]$, so that $pln p-ple qln q-q$ and hence $pln p-qln qle p-q=|p-q|$ if $0le qle ple1$.



              Next, take any real $cge1$. Note that $g(p):=pln p+cp$ (with $g(0):=0$) is convex in $pin[0,1]$. So, assuming $0le ple qle1$ and $qge e^-c$, we have
              $g(p)le g(0)vee g(q)=0vee g(q)=g(q)$ and hence
              $pln p-qln qle c(q-p)=c|p-q|$.
              Thus,
              beginequation
              pln p-qln qle c|p-q|
              endequation
              whenever $0le p,qle1$ and $qge e^-c$.



              Also, $-qln q$ is increasing in $qin[0,e^-1]$ and hence in $qin[0,e^-c]$, so that $-qln qle ce^-c$ for $qin[0,e^-c]$. Also, $pln ple0$ if $0le ple1$.



              Therefore, the difference between the entropies of $Q=(q_i)_i=1^N$ and $P=(p_i)_i=1^N$ is
              beginequation
              sum_1^N p_iln p_i-sum_1^N q_iln q_ile S_1+S_2+S_3,
              endequation
              where
              beginalign*
              S_1 &:=sum_icolon q_ige e^-c (p_iln p_i-q_iln q_i)le sum_icolon q_ige e^-cc|p_i-q_i|
              le cde quadtextif dege|P-Q|_1, \
              S_2 &:= sum_icolon q_i< e^-cp_iln p_ile0, \
              S_3 &:= sum_icolon q_i< e^-c(-q_iln q_i)lesum_icolon q_i< e^-cce^-cle Nce^-c.
              endalign*
              So, taking now $c=lnfrac Nde$ and assuming $Nge ede$, we see that
              beginequation
              sum_1^N p_iln p_i-sum_1^N q_iln q_ile
              cde+Nce^-c
              =2delnfrac Nde.
              endequation
              Taking here $de=ep/2$ and noting that $Nge1$, we conclude that
              beginequation
              sum_1^N p_iln p_i-sum_1^N q_iln q_ile
              eplnfrac2Nep
              endequation
              if $eple2/e$.






              share|cite|improve this answer


























                up vote
                1
                down vote













                $newcommanddedelta
                newcommandepepsilon$



                Note that $pln p-p$ is decreasing in $pin[0,1]$, so that $pln p-ple qln q-q$ and hence $pln p-qln qle p-q=|p-q|$ if $0le qle ple1$.



                Next, take any real $cge1$. Note that $g(p):=pln p+cp$ (with $g(0):=0$) is convex in $pin[0,1]$. So, assuming $0le ple qle1$ and $qge e^-c$, we have
                $g(p)le g(0)vee g(q)=0vee g(q)=g(q)$ and hence
                $pln p-qln qle c(q-p)=c|p-q|$.
                Thus,
                beginequation
                pln p-qln qle c|p-q|
                endequation
                whenever $0le p,qle1$ and $qge e^-c$.



                Also, $-qln q$ is increasing in $qin[0,e^-1]$ and hence in $qin[0,e^-c]$, so that $-qln qle ce^-c$ for $qin[0,e^-c]$. Also, $pln ple0$ if $0le ple1$.



                Therefore, the difference between the entropies of $Q=(q_i)_i=1^N$ and $P=(p_i)_i=1^N$ is
                beginequation
                sum_1^N p_iln p_i-sum_1^N q_iln q_ile S_1+S_2+S_3,
                endequation
                where
                beginalign*
                S_1 &:=sum_icolon q_ige e^-c (p_iln p_i-q_iln q_i)le sum_icolon q_ige e^-cc|p_i-q_i|
                le cde quadtextif dege|P-Q|_1, \
                S_2 &:= sum_icolon q_i< e^-cp_iln p_ile0, \
                S_3 &:= sum_icolon q_i< e^-c(-q_iln q_i)lesum_icolon q_i< e^-cce^-cle Nce^-c.
                endalign*
                So, taking now $c=lnfrac Nde$ and assuming $Nge ede$, we see that
                beginequation
                sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                cde+Nce^-c
                =2delnfrac Nde.
                endequation
                Taking here $de=ep/2$ and noting that $Nge1$, we conclude that
                beginequation
                sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                eplnfrac2Nep
                endequation
                if $eple2/e$.






                share|cite|improve this answer
























                  up vote
                  1
                  down vote










                  up vote
                  1
                  down vote









                  $newcommanddedelta
                  newcommandepepsilon$



                  Note that $pln p-p$ is decreasing in $pin[0,1]$, so that $pln p-ple qln q-q$ and hence $pln p-qln qle p-q=|p-q|$ if $0le qle ple1$.



                  Next, take any real $cge1$. Note that $g(p):=pln p+cp$ (with $g(0):=0$) is convex in $pin[0,1]$. So, assuming $0le ple qle1$ and $qge e^-c$, we have
                  $g(p)le g(0)vee g(q)=0vee g(q)=g(q)$ and hence
                  $pln p-qln qle c(q-p)=c|p-q|$.
                  Thus,
                  beginequation
                  pln p-qln qle c|p-q|
                  endequation
                  whenever $0le p,qle1$ and $qge e^-c$.



                  Also, $-qln q$ is increasing in $qin[0,e^-1]$ and hence in $qin[0,e^-c]$, so that $-qln qle ce^-c$ for $qin[0,e^-c]$. Also, $pln ple0$ if $0le ple1$.



                  Therefore, the difference between the entropies of $Q=(q_i)_i=1^N$ and $P=(p_i)_i=1^N$ is
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile S_1+S_2+S_3,
                  endequation
                  where
                  beginalign*
                  S_1 &:=sum_icolon q_ige e^-c (p_iln p_i-q_iln q_i)le sum_icolon q_ige e^-cc|p_i-q_i|
                  le cde quadtextif dege|P-Q|_1, \
                  S_2 &:= sum_icolon q_i< e^-cp_iln p_ile0, \
                  S_3 &:= sum_icolon q_i< e^-c(-q_iln q_i)lesum_icolon q_i< e^-cce^-cle Nce^-c.
                  endalign*
                  So, taking now $c=lnfrac Nde$ and assuming $Nge ede$, we see that
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                  cde+Nce^-c
                  =2delnfrac Nde.
                  endequation
                  Taking here $de=ep/2$ and noting that $Nge1$, we conclude that
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                  eplnfrac2Nep
                  endequation
                  if $eple2/e$.






                  share|cite|improve this answer














                  $newcommanddedelta
                  newcommandepepsilon$



                  Note that $pln p-p$ is decreasing in $pin[0,1]$, so that $pln p-ple qln q-q$ and hence $pln p-qln qle p-q=|p-q|$ if $0le qle ple1$.



                  Next, take any real $cge1$. Note that $g(p):=pln p+cp$ (with $g(0):=0$) is convex in $pin[0,1]$. So, assuming $0le ple qle1$ and $qge e^-c$, we have
                  $g(p)le g(0)vee g(q)=0vee g(q)=g(q)$ and hence
                  $pln p-qln qle c(q-p)=c|p-q|$.
                  Thus,
                  beginequation
                  pln p-qln qle c|p-q|
                  endequation
                  whenever $0le p,qle1$ and $qge e^-c$.



                  Also, $-qln q$ is increasing in $qin[0,e^-1]$ and hence in $qin[0,e^-c]$, so that $-qln qle ce^-c$ for $qin[0,e^-c]$. Also, $pln ple0$ if $0le ple1$.



                  Therefore, the difference between the entropies of $Q=(q_i)_i=1^N$ and $P=(p_i)_i=1^N$ is
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile S_1+S_2+S_3,
                  endequation
                  where
                  beginalign*
                  S_1 &:=sum_icolon q_ige e^-c (p_iln p_i-q_iln q_i)le sum_icolon q_ige e^-cc|p_i-q_i|
                  le cde quadtextif dege|P-Q|_1, \
                  S_2 &:= sum_icolon q_i< e^-cp_iln p_ile0, \
                  S_3 &:= sum_icolon q_i< e^-c(-q_iln q_i)lesum_icolon q_i< e^-cce^-cle Nce^-c.
                  endalign*
                  So, taking now $c=lnfrac Nde$ and assuming $Nge ede$, we see that
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                  cde+Nce^-c
                  =2delnfrac Nde.
                  endequation
                  Taking here $de=ep/2$ and noting that $Nge1$, we conclude that
                  beginequation
                  sum_1^N p_iln p_i-sum_1^N q_iln q_ile
                  eplnfrac2Nep
                  endequation
                  if $eple2/e$.







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                  share|cite|improve this answer








                  edited 11 mins ago

























                  answered 2 hours ago









                  Iosif Pinelis

                  14.8k12154




                  14.8k12154




















                      up vote
                      0
                      down vote













                      Claim. If $|P-Q|leqvarepsilonleqfrac12$, then $|H(P)-H(Q)| leq H(varepsilon) + varepsilonlog N$.



                      Proof.
                      Let $varepsilon':=|P-Q|$.
                      Let $(X,Y)$ be an optimal coupling of $P$ and $Q$, so that
                      beginalign
                      mathbbP(Xneq Y) = |P-Q| ;.
                      endalign
                      Using a standard construction, we can assume that $X$ and $Y$ have the particular form
                      beginalign
                      X &:=
                      begincases
                      Z & textif $B=0$, \
                      tildeX & textif $B=1$,
                      endcases &
                      Y &:=
                      begincases
                      Z & textif $B=0$, \
                      tildeY & textif $B=1$,
                      endcases
                      endalign
                      where $B$, $Z$ and $(tildeX,tildeY)$ are independent and $BsimtextBern(varepsilon')$.



                      Note that
                      beginalign
                      H(X|B) leq H(X) leq H(B) + H(X|B) ;.
                      endalign
                      For $H(X|B)$ we can write
                      beginalign
                      H(X|B) &= varepsilon' H(X|B=1) + (1-varepsilon') H(X|B=0) \
                      &= varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;.
                      endalign
                      Thus,
                      beginalign
                      varepsilon' H(tildeX) + (1-varepsilon') H(Z) &leq H(X)
                      leq H(B) + varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;,
                      tag$clubsuit$
                      endalign
                      and similarly,
                      beginalign
                      varepsilon' H(tildeY) + (1-varepsilon') H(Z) &leq H(Y)
                      leq H(B) + varepsilon' H(tildeY) + (1-varepsilon') H(Z) ;.
                      tag$spadesuit$
                      endalign



                      Combining ($clubsuit$) and ($spadesuit$) we get
                      beginalign
                      |H(X)-H(Y)| &leq
                      H(B) + varepsilon' |H(tildeX) - H(tildeY)| \
                      &leq H(varepsilon') + varepsilon' log N \
                      &leq H(varepsilon) + varepsilon log N ;,
                      endalign
                      as claimed.
                      QED






                      share|cite|improve this answer
























                        up vote
                        0
                        down vote













                        Claim. If $|P-Q|leqvarepsilonleqfrac12$, then $|H(P)-H(Q)| leq H(varepsilon) + varepsilonlog N$.



                        Proof.
                        Let $varepsilon':=|P-Q|$.
                        Let $(X,Y)$ be an optimal coupling of $P$ and $Q$, so that
                        beginalign
                        mathbbP(Xneq Y) = |P-Q| ;.
                        endalign
                        Using a standard construction, we can assume that $X$ and $Y$ have the particular form
                        beginalign
                        X &:=
                        begincases
                        Z & textif $B=0$, \
                        tildeX & textif $B=1$,
                        endcases &
                        Y &:=
                        begincases
                        Z & textif $B=0$, \
                        tildeY & textif $B=1$,
                        endcases
                        endalign
                        where $B$, $Z$ and $(tildeX,tildeY)$ are independent and $BsimtextBern(varepsilon')$.



                        Note that
                        beginalign
                        H(X|B) leq H(X) leq H(B) + H(X|B) ;.
                        endalign
                        For $H(X|B)$ we can write
                        beginalign
                        H(X|B) &= varepsilon' H(X|B=1) + (1-varepsilon') H(X|B=0) \
                        &= varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;.
                        endalign
                        Thus,
                        beginalign
                        varepsilon' H(tildeX) + (1-varepsilon') H(Z) &leq H(X)
                        leq H(B) + varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;,
                        tag$clubsuit$
                        endalign
                        and similarly,
                        beginalign
                        varepsilon' H(tildeY) + (1-varepsilon') H(Z) &leq H(Y)
                        leq H(B) + varepsilon' H(tildeY) + (1-varepsilon') H(Z) ;.
                        tag$spadesuit$
                        endalign



                        Combining ($clubsuit$) and ($spadesuit$) we get
                        beginalign
                        |H(X)-H(Y)| &leq
                        H(B) + varepsilon' |H(tildeX) - H(tildeY)| \
                        &leq H(varepsilon') + varepsilon' log N \
                        &leq H(varepsilon) + varepsilon log N ;,
                        endalign
                        as claimed.
                        QED






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                          up vote
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                          down vote









                          Claim. If $|P-Q|leqvarepsilonleqfrac12$, then $|H(P)-H(Q)| leq H(varepsilon) + varepsilonlog N$.



                          Proof.
                          Let $varepsilon':=|P-Q|$.
                          Let $(X,Y)$ be an optimal coupling of $P$ and $Q$, so that
                          beginalign
                          mathbbP(Xneq Y) = |P-Q| ;.
                          endalign
                          Using a standard construction, we can assume that $X$ and $Y$ have the particular form
                          beginalign
                          X &:=
                          begincases
                          Z & textif $B=0$, \
                          tildeX & textif $B=1$,
                          endcases &
                          Y &:=
                          begincases
                          Z & textif $B=0$, \
                          tildeY & textif $B=1$,
                          endcases
                          endalign
                          where $B$, $Z$ and $(tildeX,tildeY)$ are independent and $BsimtextBern(varepsilon')$.



                          Note that
                          beginalign
                          H(X|B) leq H(X) leq H(B) + H(X|B) ;.
                          endalign
                          For $H(X|B)$ we can write
                          beginalign
                          H(X|B) &= varepsilon' H(X|B=1) + (1-varepsilon') H(X|B=0) \
                          &= varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;.
                          endalign
                          Thus,
                          beginalign
                          varepsilon' H(tildeX) + (1-varepsilon') H(Z) &leq H(X)
                          leq H(B) + varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;,
                          tag$clubsuit$
                          endalign
                          and similarly,
                          beginalign
                          varepsilon' H(tildeY) + (1-varepsilon') H(Z) &leq H(Y)
                          leq H(B) + varepsilon' H(tildeY) + (1-varepsilon') H(Z) ;.
                          tag$spadesuit$
                          endalign



                          Combining ($clubsuit$) and ($spadesuit$) we get
                          beginalign
                          |H(X)-H(Y)| &leq
                          H(B) + varepsilon' |H(tildeX) - H(tildeY)| \
                          &leq H(varepsilon') + varepsilon' log N \
                          &leq H(varepsilon) + varepsilon log N ;,
                          endalign
                          as claimed.
                          QED






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                          Claim. If $|P-Q|leqvarepsilonleqfrac12$, then $|H(P)-H(Q)| leq H(varepsilon) + varepsilonlog N$.



                          Proof.
                          Let $varepsilon':=|P-Q|$.
                          Let $(X,Y)$ be an optimal coupling of $P$ and $Q$, so that
                          beginalign
                          mathbbP(Xneq Y) = |P-Q| ;.
                          endalign
                          Using a standard construction, we can assume that $X$ and $Y$ have the particular form
                          beginalign
                          X &:=
                          begincases
                          Z & textif $B=0$, \
                          tildeX & textif $B=1$,
                          endcases &
                          Y &:=
                          begincases
                          Z & textif $B=0$, \
                          tildeY & textif $B=1$,
                          endcases
                          endalign
                          where $B$, $Z$ and $(tildeX,tildeY)$ are independent and $BsimtextBern(varepsilon')$.



                          Note that
                          beginalign
                          H(X|B) leq H(X) leq H(B) + H(X|B) ;.
                          endalign
                          For $H(X|B)$ we can write
                          beginalign
                          H(X|B) &= varepsilon' H(X|B=1) + (1-varepsilon') H(X|B=0) \
                          &= varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;.
                          endalign
                          Thus,
                          beginalign
                          varepsilon' H(tildeX) + (1-varepsilon') H(Z) &leq H(X)
                          leq H(B) + varepsilon' H(tildeX) + (1-varepsilon') H(Z) ;,
                          tag$clubsuit$
                          endalign
                          and similarly,
                          beginalign
                          varepsilon' H(tildeY) + (1-varepsilon') H(Z) &leq H(Y)
                          leq H(B) + varepsilon' H(tildeY) + (1-varepsilon') H(Z) ;.
                          tag$spadesuit$
                          endalign



                          Combining ($clubsuit$) and ($spadesuit$) we get
                          beginalign
                          |H(X)-H(Y)| &leq
                          H(B) + varepsilon' |H(tildeX) - H(tildeY)| \
                          &leq H(varepsilon') + varepsilon' log N \
                          &leq H(varepsilon) + varepsilon log N ;,
                          endalign
                          as claimed.
                          QED







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                          answered 33 mins ago









                          Algernon

                          8691712




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