theorem Th19:
  seq is convergent & (ex k st for n st k<=n holds seq1.n=seq.n)
  implies seq1 is convergent
proof
  assume that
A1: seq is convergent and
A2: ex k st for n st k<=n holds seq1.n=seq.n;
  consider g1 being Complex such that
A3: for p st 0<p ex n st for m st n<=m holds |.(seq.m)-g1.|<p by A1;
  consider k such that
A4: for n st k<=n holds seq1.n=seq.n by A2;
  take g=g1;
  let p;
  assume 0<p;
  then consider n1 such that
A5: for m st n1<=m holds |.(seq.m)-g1.|<p by A3;
A6: now
    assume
A7: n1<=k;
    take n=k;
    let m;
    assume
A8: n<=m;
    then n1<=m by A7,XXREAL_0:2;
    then |.(seq.m)-g1.|<p by A5;
    hence |.(seq1.m)-g.|<p by A4,A8;
  end;
  now
    assume
A9: k<=n1;
    take n=n1;
    let m;
    assume
A10: n<=m;
    then seq1.m=seq.m by A4,A9,XXREAL_0:2;
    hence |.(seq1.m)-g.|<p by A5,A10;
  end;
  hence ex n st for m st n<=m holds |.seq1.m-g.|<p by A6;
end;
